Leary v. United States, 395 U.S. 6 (1969), is a U.S. Supreme Court case dealing with the constitutionality of the Marihuana Tax Act of 1937.

The unanimous opinion of the court was penned by Justice John Marshall Harlan II and declared the Marihuana Tax Act unconstitutional.

They halted at the American secondary inspection area, explained the situation to a customs inspector, and stated that they had nothing from Mexico to declare.

If read according to its terms, the Marijuana Tax Act compelled petitioner to expose himself to a "real and appreciable" risk of self-incrimination; b.

[The statute] required him, in the course of obtaining an order form, to identify himself not only as a transferee of marijuana but as a transferee who had not registered and paid the occupational tax; c. Compliance with the transfer tax provisions would have required petitioner unmistakably to identify himself as a member of [a]..."selective" and "suspect" group, we can only decide that when read according to their terms these provisions created a "real and appreciable" hazard of incrimination.