Rosenthal's case represented the latest clash between state and federal authorities over the medical use of marijuana. The federal government does not recognize medical marijuana laws in the nine states, including California, that have them.
In February, a jury concluded Rosenthal was growing more than 100 plants, conspired to cultivate marijuana, and maintained an Oakland warehouse for a growing operation.
Rosenthal, 58, had said he was acting as an agent for Oakland's medical marijuana program, an outgrowth of a 1996 medical marijuana initiative approved by California's voters.
But jurors were not allowed to hear those arguments, and several of them later said they would have acquitted Rosenthal had they known.
On Wednesday, U.S. District Judge Charles Breyer sentenced Rosenthal to one day in prison on each of three counts, to run concurrently, and then set him free after declaring Rosenthal had already served that time. The ruling was met by cheering and applause in the courtroom.
The ruling was met by cheering and applause in the courtroom.
"I think it's a marvelous victory for states' rights and the medical use of marijuana," said Keith Stroup, executive director of the National Organization for the Reform of Marijuana Laws. "It sends a strong signal to the federal government that they should reconsider their current program of arresting patients and caregivers in California."
Rosenthal also was fined $1,000 and will be on supervised release for three years.
"I take responsibility for my actions that bring me here today. I took these actions because my conscience led me to help people who are suffering," Rosenthal said outside the courtroom. "These laws are doomed."
Last week, California Attorney General Bill Lockyer asked the judge for leniency in Rosenthal's sentencing. Federal prosecutors had asked for a 6@1/2-year prison term.
Prosecutor George Bevan said that Rosenthal was not simply helping the ill.
"This operation is a cash cow. He put out thousands and thousands of plants," Bevan said. "I don't think anyone disagrees with helping sick people, but as far as we're concerned, it was a business."
Federal law does not permit legalization of marijuana for medical use, although Alaska, Arizona, California, Colorado, Hawaii, Maine, Nevada, Oregon and Washington allow it.