XAVIER ALVAREZ IS a serial liar. He lied, as his lawyers concede, about playing in the National Hockey League. He lied about being wounded in the course of rescuing the American ambassador during the Iran hostage crisis. He lied when he told a California audience that he had been awarded the Congressional Medal of Honor.

The first two prevarications made Mr. Alvarez unquestionably untrustworthy or, as some of his critics put it, a “jerk” and an “idiot.” The claim about the medal made him a criminal.

After being outed as a fibber in the media, Mr. Alvarez was charged by the Justice Department with violating the Stolen Valor Act of 2005, which targets an individual who “falsely represents himself or herself, verbally or in writing, to have been awarded any decoration or medal authorized by Congress for the Armed Forces of the United States.” The law was intended to preserve the value of these dearly won honors against depreciation by imposters.

Mr. Alvarez challenged the constitutionality of the act, arguing that it violates the First Amendment by giving the government expansive and impermissible power to regulate the content of speech. The U.S. Court of Appeals for the 9th Circuit — a favorite punching bag of the Supreme Court — agreed with Mr. Alvarez; the Supreme Court justices are scheduled to hear arguments on Wednesday. They should hold their noses and side with Mr. Alvarez. (Disclosure: The Washington Post joined a group of media organizations in filing a brief in support of Mr. Alvarez.)

In defending the Stolen Valor Act, the Obama administration argues that the First Amendment was never meant as a blanket protection for all manner of speech. It points to the fact that the government already has the power to prosecute certain forms of speech that include obscenity and fraud. We would add, as justifiably criminal, perjury or lying to law enforcement officials in the course of an investigation, given the government’s interest in ensuring justice. Individuals also may bring legal action when they believe they have been defamed or libeled through false statements.

Mr. Alvarez’s boastful deceits do not fall into any of these narrow categories; his are pure — if ignoble — expressions of speech. To allow the government to become the ultimate arbiter of truth would set a terrible precedent that could erode the First Amendment’s guarantee of freedom of speech and potentially ensnare mere braggarts who do no harm other than to their own credibility.