In Smith v. Maryland, 442 U.S. 735 (1979), the Supreme Court held that police do not need a warrant to record the phone numbers a person dials, because those numbers are voluntarily turned over to the telephone company and no one has a reasonable expectation of privacy in information handed to a third party. The decision established what is now called the third-party doctrine, and it remains the foundation for warrantless government access to phone logs, email routing data, IP records, and other business records nearly fifty years later.
The Case Behind the Ruling
Patricia McDonough was robbed in Baltimore on March 5, 1976. In the days after, she began receiving threatening and obscene phone calls from a man who claimed to be the robber. She gave police a description of the caller and of a car she had seen near her home.1Oyez. Smith v. Maryland
On March 16, officers spotted a 1975 Monte Carlo matching her description and traced it to Michael Lee Smith. The next day, without a warrant or court order, police asked the telephone company to install a pen register at its central office to record the numbers dialed from Smith’s home phone.2Open Casebook. Smith v. Maryland
A pen register logs outgoing numbers but does not capture the content of any conversation. The device confirmed a call from Smith’s line to McDonough’s home. Police used that result to get a warrant, searched Smith’s house, and found more incriminating evidence. Smith moved to suppress the pen register data, arguing the warrantless installation violated the Fourth Amendment.1Oyez. Smith v. Maryland
What the Court Held
The Court analyzed the pen register under the two-part test from Katz v. United States (1967): a government action counts as a Fourth Amendment search only if the person had an actual expectation of privacy and society recognizes that expectation as reasonable.3Constitution Annotated. Amdt4.3.3 Katz and Reasonable Expectation of Privacy Test
The majority found Smith failed on both prongs. Phone users know the company records dialed numbers for billing — customers see them every month on their statements. That awareness, the Court reasoned, undercuts any subjective belief that dialed numbers are private. And even if Smith personally believed they were, society would not treat that belief as reasonable, because dialing transmits information to the company’s switching equipment as a matter of course.4Justia U.S. Supreme Court Center. Smith v. Maryland, 442 U.S. 735 (1979)
From that finding came the broader rule. When you voluntarily convey information to a third party, you assume the risk it will be shared with the government, and you lose Fourth Amendment protection over it. The purpose of the disclosure does not matter — Smith shared the numbers to complete calls, not for law enforcement, but the act of disclosure was enough.
The Court had reached a parallel conclusion three years earlier about bank records in United States v. Miller, 425 U.S. 435 (1976), holding that a customer has no Fourth Amendment interest in checks and deposit slips held by a bank.5Oyez. United States v. Miller Together, Smith and Miller became the twin pillars of the third-party doctrine. Because the pen register captured information the phone company already held as its own business records, no search occurred, no probable cause was required, and no judge had to sign off.4Justia U.S. Supreme Court Center. Smith v. Maryland, 442 U.S. 735 (1979)
The Dissents
Two dissenting opinions pushed back, each on different ground.
Justice Stewart’s Dissent
Justice Stewart, joined by Justice Brennan, argued that dialed numbers deserve the same protection as the conversations they enable. You cannot place a call without sending numbers through the company’s equipment, just as you cannot speak without the company’s wires carrying your voice. If Katz protects the call, Stewart wrote, it should protect the dialing that makes the call possible. What the company does with those numbers for billing “is no more relevant to this inquiry than it would be in a case involving the conversation itself.”4Justia U.S. Supreme Court Center. Smith v. Maryland, 442 U.S. 735 (1979)
Justice Marshall’s Dissent
Justice Marshall, also joined by Brennan, attacked the third-party doctrine at its root. Calling telephone use “voluntary,” he wrote, ignores that the phone system is a necessity of modern life. The real choice is to accept the company’s terms or forgo the technology entirely, and the Constitution should not force that trade.
Marshall also warned about what a pen register reveals. Even without a word of conversation, a log of dialed numbers exposes intimate patterns: who you call, how often, when. That record can identify your doctor, your lawyer, your political ties, and your close relationships. The Fourth Amendment, he argued, should reach those associational patterns even when they pass through a third-party switchboard.6FindLaw. Smith v. Maryland, 442 U.S. 735 (1979)
What Congress Did Afterward
The Constitution did not require a warrant for pen registers after Smith, but Congress imposed some oversight anyway. As part of the Electronic Communications Privacy Act of 1986, the Pen Register Act (18 U.S.C. §§ 3121–3127) made it a crime to install or use a pen register without a court order.7Office of the Law Revision Counsel. 18 USC 3121 – General Prohibition on Pen Register and Trap and Trace Device Use
The protection is thinner than a warrant. A government attorney only has to certify that the information sought is “relevant to an ongoing criminal investigation.” The judge does not weigh evidence or assess probable cause. If the certification is made, the statute says the court “shall” issue the order.8Office of the Law Revision Counsel. 18 USC 3123 – Issuance of an Order for a Pen Register or a Trap and Trace Device The Act also has no exclusionary rule, so illegally obtained pen register data may still be admissible, and the person surveilled has no private right of action.
How the Rule Applies to Email, Internet, and Bulk Data
Smith involved a rotary-era phone, but courts have carried its logic into digital communications. In United States v. Forrester, the Ninth Circuit held that government surveillance of email header information and the IP addresses of websites a person visits is not a Fourth Amendment search. The court called those techniques “constitutionally indistinguishable” from the pen register in Smith, because internet users transmit routing data to their providers to complete communications. Addressing information shows whom you contacted, not what you said.
The doctrine has also been invoked to justify far larger programs. The National Security Agency’s bulk collection of telephone metadata, revealed in 2013, relied in part on Smith‘s premise that call records held by carriers sit outside Fourth Amendment protection. Justice Marshall’s warnings about government tracking of associational patterns had, by then, materialized at a scale the 1979 Court did not anticipate.
Where Carpenter Drew a Line
In 2018, the Supreme Court placed its first real limit on the third-party doctrine. Carpenter v. United States involved 127 days of cell-site location information that police obtained from wireless carriers without a warrant. In a 5–4 ruling, the Court held that accessing this data was a Fourth Amendment search and required a warrant supported by probable cause.9Justia U.S. Supreme Court Center. Carpenter v. United States, 585 U.S. ___ (2018)
The Court distinguished Smith on two grounds. Location tracking produces an “exhaustive chronicle” of a person’s movements and reveals deeply private associations in a way a list of dialed numbers does not. And the “voluntary disclosure” idea breaks down for cell phones, which log their location automatically whenever powered on, without any affirmative act by the user. Carrying a phone, the Court said, is “indispensable to participation in modern society.”10Supreme Court of the United States. Carpenter v. United States
The majority called its ruling “narrow” and left Smith and Miller in place for the records those cases addressed. Dialed phone numbers and bank records remain unprotected. But Carpenter established that the doctrine has limits when surveillance technology assembles a comprehensive picture of someone’s private life.
Why It Still Matters
Smith draws the line that runs through nearly every modern surveillance debate: content versus metadata. The words you speak are protected. The data about whom you called, when, and for how long is not. Whenever the government seeks phone records, email headers, IP logs, or other routing data without a warrant, Smith is the foundational authority, and its core holding remains intact.
The tension the dissenters flagged has only grown. In 1979 a pen register captured a short list of numbers from a single line. Today, communications metadata can map a person’s entire social network, daily routine, and physical movements. Whether the third-party doctrine can survive the distance between its 1970s assumptions and the realities of digital life is a question Smith itself cannot answer, but one every future privacy case has to confront.