WASHINGTON -- Seriously ill people who use marijuana to ease their pain can be prosecuted for violating federal law even in states where its medical use is legal, the U.S. Supreme Court ruled yesterday.

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By a 6-3 vote, the court held that two women in California could not escape federal prosecution for using marijuana under a doctor's prescription even though California allows such use and the marijuana they used was grown entirely inside the state.

The ruling was a personal defeat for Angel McClary Raich, who suffers from an inoperable brain tumor, fibromyalgia, endometriosis, scoliosis and uterine fibroid tumors, and Diane Monson, who has degenerative spine disease. Both reacted yesterday with defiance.

"I'm just going to keep doing what I'm doing. I don't really have a choice but to, because if I stop using cannabis, I would die," said Raich, of Oakland, Calif. She says she smokes marijuana every few hours.

Monson, an accountant who lives near Oroville, Calif., grows her own marijuana. "I'm going to have to be prepared to be arrested," she told The Associated Press.

The Bush administration has taken a tough stand against state medical marijuana laws, but it was unclear yesterday how it would respond to its new prosecutorial power. Justice Department spokesman John Nowacki would not say whether prosecutors would pursue cases against individual users. Local and state officers handle nearly all marijuana prosecutions.

The American Civil Liberties Union, which sided with the two women, saw a ray of hope in the fact that patients in California and nine other states are still protected against state prosecution.

"The power of state governments to enact and enforce state medical marijuana laws is not affected by the Supreme Court's ruling," said Allen Hopper, an attorney with the ACLU's Drug Law Reform Project.

The ruling does not strike down California's law, or similar ones in Alaska, Colorado, Hawaii, Maine, Montana, Nevada, Oregon, Vermont and Washington state. However, it may hurt efforts to pass similar laws in other states.

John Walters, director of national drug control policy, defended the government's ban. "Science and research have not determined that smoking marijuana is safe or effective," he said.

California's law, passed by voters in 1996, allows people to grow, smoke or obtain marijuana for medical needs with a doctor's recommendation. Monson and Raich contend that traditional medicines do not provide the relief that marijuana does.

Yesterday's decision was seen by legal scholars as arresting -- if not reversing -- a recent trend in which the high court has limited the power of Congress to regulate interstate commercial transactions.

Writing for the majority, Justice John Paul Stevens said the issue was not whether marijuana serves a medical purpose but whether Congress acted within its constitutional power to regulate interstate commerce when it outlawed any use of marijuana.

Citing a 1942 case in which the court upheld a federal law rationing the production of wheat even when grown for home use, Stevens wrote: "In both cases, the regulation is squarely within Congress' commerce power because production of the commodity for home consumption, be it wheat or marijuana, has a substantial effect on supply and demand in the national market for that commodity."

Stevens' majority opinion was signed by the court's other liberals -- David H. Souter, Ruth Bader Ginsburg and Stephen Breyer -- and by the moderately conservative Anthony Kennedy.

Conservative Antonin Scalia concurred in the result, but filed his own opinion offering what he called a "more nuanced" analysis. Scalia wrote that congressional authority to legislate against the use of marijuana was grounded not only in the Constitution's Commerce Clause but also in language allowing Congress to pass laws "necessary and proper" to carry out its responsibilities.

Kennedy's and Scalia's presence in the majority was somewhat surprising because both were members of 5-4 majorities in two landmark decisions scaling back Congress' authority under the Commerce Clause. The other members of the so-called "federalism five" were Chief Justice William H. Rehnquist and Justices Sandra day O'Connor and Clarence Thomas.

In the 1995 case of U.S. v. Lopez, those five justices struck down the federal Gun-Free School Zones Act because banning firearms near schools had only an "indirect and remote" connection to interstate commerce.

In a 2000 case called U.S. v. Morrison, the same 5-4 majority invalidated a section of the federal Violence against Women Act that allowed victims of sexual abuse to file suit in federal court. The court in that case said that "gender-motivated crimes of violence are not, in any sense, economic activity."

Yesterday three members of the Lopez and Morrison majorities complained that the court had cut back on the state's-right principles that decided those cases.

O'Connor's complaint that the court was undermining its own "federalism revolution" received some support from academic observers.

Arthur Hellman, a professor at the University of Pittsburgh School of Law, said it will take additional cases to determine if the court is retrenching on its commitment to state's rights. But he said Scalia and Kennedy had opened themselves to suspicion that they were abandoning the principles of the Lopez and Morrison decisions in a case in which consistency would have required them to approve of the use of marijuana under state law, albeit for medical purposes.

"In the past, Kennedy has written eloquently about a limited national government, so it's disappointing that he didn't write separately to explain why he joined the majority," Hellman said. "As for Scalia, it was puzzling that his opinion says nothing about the original intent of the framers of the Constitution as he does when he writes about religion and the First Amendment."