The 1937 federal marijuana tax act was struck down by the Supreme Court
in 1969. In a case brought by Timothy Leary, the Court held that the
law's requirement that a would-be possessor of marijuana register with
the local bureau of the IRS, thereby placing his name and address on a
file available to local law enforcment, violated the Fifth Amendment
privilege against self-incrimination, given the fact that at the time
all 50 states had state laws on the books outlawing marijuana outright.
In 1970, the Controlled Substances Act made possession of marijuana
illegal again on a federal level, without the constitutional issues
that scuttled the 1937 act. Several petitions for cannabis rescheduling
in the United States have been filed, since the Act permits
legalization of marijuana through the executive branch.

http://en.wikipedia.org/wiki/Cannabis_%28drug%29