MR. JUSTICE BLACKMUN delivered the opinion of the Court.
On March 5, 1976, in Baltimore, Md. Patricia McDonough was robbed. 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 ... After the robbery, McDonough began receiving threatening and obscene phone calls ... By tracing the license plate number, police learned that the car was registered in the name of petitioner, Michael Lee Smith.
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 ...The police did not get a warrant or court order ... The register revealed that, on March 17, a call was placed from petitioner's home to McDonough's phone ... On the basis of this and other evidence, the police obtained a warrant to search petitioner's residence ... Petitioner was arrested, and a six-man lineup was held on March 19. McDonough identified petitioner as the man who had robbed her ...
... By pretrial motion, he sought to suppress "all fruits derived from the pen register" ... The trial court denied the suppression motion, holding that the warrantless installation of the pen register did not violate the Fourth Amendment ...
The Court of Appeals affirmed the judgment of conviction ...
... Because there was no "search," the court concluded, no warrant was needed. Three judges dissented ...
Consistently with Katz, this Court uniformly has held that the application of the Fourth Amendment depends on whether the person invoking its protection can claim a "justifiable," a "reasonable," or a "legitimate expectation of privacy" ...
... Since the pen register was installed on telephone company property at the telephone company's central offices, petitioner obviously cannot claim that his "property" was invaded or that police intruded into a "constitutionally protected area" ...
... All telephone users realize that they must "convey" phone numbers to the telephone company, since it is through telephone company switching equipment that their calls are completed. All subscribers realize, moreover, that the phone company has facilities for making permanent records of the numbers they dial, for they see a list of their long-distance (toll) calls on their monthly bills ... Although most people may be oblivious to a pen register's esoteric functions, they presumably have some awareness of one common use: to aid in the identification of persons making annoying or obscene calls ...
... When he used his phone, petitioner voluntarily conveyed numerical information to the telephone company and "exposed" that information to its equipment in the ordinary course of business. In so doing, petitioner assumed the risk that the company would reveal to police the numbers he dialed ...
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