Chief Justice William H. Rehnquist (search) has been hospitalized with a fever, a setback that fueled more retirement speculation about the 33-year veteran of the Supreme Court.

The 80-year-old Rehnquist was taken by ambulance to Virginia Hospital Center in Arlington, Va., on Tuesday night and was admitted for observation and tests, Supreme Court spokeswoman Kathy Arberg said Wednesday. She would not release other information, including his condition.

Despite having thyroid cancer, Rehnquist has maintained a regular work routine and defied expectations that bad health would force him to leave the court.

"This hospitalization has to shake his faith a little bit," said Stephen Wermiel, an American University law professor who specializes in the Supreme Court.

President Bush was unaware of Rehnquist's hospitalization. He was in the Oval Office when Andy Card, the White House chief of staff, and spokesman Scott McClellan informed him at midafternoon of news reports that Rehnquist was ill.

There was no indication that the news would affect the president's selection of a candidate — or the timing of an announcement — for replacing Justice Sandra Day O'Connor (search), who announced earlier this month that she was stepping down.

Rehnquist, who began receiving chemotherapy and radiation in October, has said very little publicly about his prognosis and nothing about his future at the court.

A month ago, most court watchers thought Rehnquist's retirement was inevitable. But many justices in history have kept working until the end. There have been just 15 previous chief justices, and eight served until their deaths.

"I understand why he wouldn't want to retire. If he quit he'd feel like he was taking one step in the grave," said Richard Friedman, a law professor at the University of Michigan. "He's probably enjoying all the fuss."

Camera crews were staked outside the hospital Wednesday evening. It was unclear whether the fever that put Rehnquist there was related to his cancer.

Senate Judiciary Committee Chairman Arlen Specter, who has a type of cancer involving the lymph nodes, said doctors "take extensive precautions with cancer patients who have elevated symptoms."

The chief justice had been coming to the court daily but did not show up as usual Wednesday morning. Court officials initially declined to say why he was absent or explain unusual happenings at Rehnquist's Arlington home.

Members of the news media near the residence saw a court police officer make several trips to the house, leaving each time with various personal items. First,the officer carried out Rehnquist's distinctive cane and a shirt. Later, he brought out other clothing.

At about 2:30 p.m., Arberg issued a two-sentence statement saying Rehnquist was hospitalized for a fever and was undergoing tests.

It was the second time in less than four months that Rehnquist was taken by ambulance to the hospital. In March, he was brought in with breathing problems. He did not stay overnight then.

Rehnquist has a tracheotomy tube that helps him breathe. He has been treated since October for thyroid cancer, undergoing chemotherapy and radiation. The illness led to a five-month absence from the bench, although he continued working at home and at the court during his convalescence.

The chief justice has refused to say if he plans to retire, telling reporters camped outside his house last week: "That's for me to know and you to find out."

Dr. Kenneth Burman, a thyroid specialist at Washington Hospital Center who is not involved in Rehnquist's treatment, said it was not unusual for a cancer patient to check into the hospital with fever. "It could be a minor local infection around the tracheotomy tube," he said.

A more serious possibility, he said, is that the cancer has spread and caused infection. Other possibilities, he said, are pneumonia, allergies to medicine or reaction to chemotherapy.

So far, the Supreme Court has released only the barest of details about the chief justice's health. Among the unanswered questions is whether Rehnquist has the most serious type of thyroid cancer, which is often fatal within months.