Washington ? The Supreme Court returns Monday with an ailing leader, pressure to rule quickly on the constitutionality of federal prison sentences and a slew of contentious issues to decide, from medical marijuana to Ten Commandment displays.

The most immediate concern is the health of Chief Justice William Rehnquist, who was diagnosed with thyroid cancer in October. He has been working part time at the court for more than two weeks but still is too ill to return to the bench.

At 80, Rehnquist already was considered a top retirement prospect on a court that has had no turnover in a decade, a record. But justices try to time their departures for the court’s summer recess, to avoid 4-4 ties in cases.

Rehnquist’s absence has yet to produce any tie votes. Even though he missed all the arguments in November and December, he has reviewed the cases and could vote if needed.

“I suspect no matter what his health, he’ll do whatever he can to hang in there,” said Stephen McAllister, law school dean at Kansas University and a former Supreme Court clerk. “There is nothing that frustrates them more than an eight-person court.”

Besides Rehnquist, Justices John Paul Stevens, 84, and Sandra Day O’Connor, 74, are considered possibilities to leave this year.

McAllister predicted court vacancies “will be the big story of 2005.”

But there will be others.

Justices could rule as early as this week on whether the longtime system for sentencing federal defendants should be thrown out because it lets judges, rather than juries, decide factors that add years to prison time. Many judges have delayed sentencings while awaiting the high court’s decision.

Other major cases involve whether states can execute juvenile killers, whether the federal government can prosecute people who use marijuana medicinally, and whether states can bar interstate wine sales over the Internet.