442 U.S. 735 (1979)
On March 5, 1976, Patricia McDonough was robbed in Baltimore, Maryland.1 She gave the police a description of the robber and of a 1975 Monte Carlo automobile she had observed near the scene of the crime.2 After the robbery, McDonough began receiving threatening and obscene phone calls from a man identifying himself as the robber.3 On one occasion, the caller asked that she step out on her front porch, and she saw the 1975 Monte Carlo moving slowly past her home.4
On March 16, police spotted a man who met McDonough's description driving a 1975 Monte Carlo in her neighborhood.5 By tracing the license plate number, police learned that the car was registered in the name of petitioner Michael Lee Smith.6 The next day, the telephone company, at police request, installed a pen register at its central offices to record the numbers dialed from the telephone at petitioner's home without obtaining a warrant or court order.7
The pen register revealed that on March 17 a call was placed from petitioner's home to McDonough's phone.8 On the basis of this and other evidence, the police obtained a warrant to search petitioner's residence.9 The search revealed that a page in petitioner's phone book was turned down to the name and number of Patricia McDonough, which was seized.10 Petitioner was arrested, and McDonough identified him as the robber in a lineup on March 19.11
Petitioner was indicted for robbery in the Criminal Court of Baltimore.12 By pretrial motion, he sought to suppress all fruits derived from the pen register.13 The trial court denied the suppression motion.14 Petitioner waived a jury, and the case was submitted to the court on an agreed statement of facts.15 The pen register tape and the phone book were admitted into evidence against him.16
Petitioner was convicted and sentenced to six years.17 He appealed to the Maryland Court of Special Appeals, but the Court of Appeals of Maryland issued a writ of certiorari to the intermediate court and affirmed the judgment of conviction.18 The United States Supreme Court granted certiorari.19
Whether the installation and use of a pen register constitutes a search within the meaning of the Fourth Amendment?20
The Fourth Amendment guarantees the right of the people to be secure in their persons, houses, papers, and effects against unreasonable searches and seizures.21 In determining whether government-initiated electronic surveillance constitutes a search, the lodestar is Katz v. United States, which holds that the Fourth Amendment protects people, not places, and that a search occurs when the government violates a justifiable expectation of privacy.22 This inquiry normally embraces two discrete questions: first, whether the individual has exhibited an actual subjective expectation of privacy by seeking to preserve something as private; and second, whether the individual's subjective expectation of privacy is one that society is prepared to recognize as reasonable or justifiable under the circumstances.23
No. Applying the Katz analysis to the facts of this case, the petitioner cannot claim a legitimate expectation of privacy in the numbers he dialed.24 The activity challenged is the installation and use of a pen register at the telephone company's central offices, which does not acquire the contents of communications but only the numbers dialed.25 Telephone users realize that they must convey phone numbers to the telephone company to complete calls and that the company has facilities for recording those numbers, as evidenced by the listing of long-distance calls on monthly bills and the routine use of pen registers for billing, fraud detection, and other purposes.26 Even if the petitioner had some subjective expectation of privacy in the numbers dialed from his home phone, that expectation is not one society is prepared to recognize as reasonable.27
A person has no legitimate expectation of privacy in information he voluntarily turns over to third parties.28 The petitioner voluntarily conveyed the numerical information to the telephone company when he placed the call, assuming the risk that the company would reveal the numbers to the police, just as a bank depositor assumes the risk that financial information will be conveyed to the government.29 The fortuity of whether the phone company records local calls does not alter this analysis, as the petitioner had facilities for recording and was free to record the information.30 Therefore, the installation and use of the pen register was not a search, and no warrant was required before obtaining the evidence that led to the petitioner's conviction for robbery.31
The installation and use of a pen register to record the numbers dialed from a person's telephone does not constitute a search within the meaning of the Fourth Amendment.32 There is no legitimate expectation of privacy in those numbers.
Related opinions on this issue
Justice Stewart dissented from the majority's conclusion.33 He argued that the numbers dialed from a private telephone fall within the constitutional protection of the Fourth and Fourteenth Amendments as recognized in Katz.34 Stewart emphasized that the vital role of the private telephone in personal communications means that the numbers dialed, like the conversations themselves, are entitled to protection.35
He rejected the notion that the telephone company's possible recording of numbers for billing purposes removes the expectation of privacy, noting that the conversation itself is protected despite being transmitted by company equipment.36 Stewart concluded that the information obtained by pen register surveillance is information in which the telephone subscriber has a legitimate expectation of privacy.37 The information captured by such surveillance emanates from private conduct within the home and is an integral part of the protected telephonic communication.38
Justice Marshall also dissented, maintaining that constitutional protections are not lost whenever a person imparts information to another for a limited purpose.39 He challenged the majority's finding of no subjective expectation of privacy, arguing that individuals do not expect information given to a phone company for business purposes to be made available to the government.40
Marshall further contended that the assumption of risk analysis is flawed because people have no realistic choice but to use the telephone, a modern necessity, and thus cannot be said to assume the risk of surveillance.41 He asserted that the reasonableness of privacy expectations should be determined by the risks society should force individuals to assume in a free society.42 Unfettered use of pen registers would impede political affiliation and journalistic endeavors by allowing government monitoring of personal contacts without probable cause or a warrant.43