WASHINGTON — In a voice barely an echo of his booming courtroom baritone, William Rehnquist, 80, yesterday administered the presidential oath of office for the fifth time in his 19-year reign as chief justice of the U.S. Supreme Court.

But with the chief justice arriving late and leaving early, many eyes were on his slow, cane-aided gait.

A dark cap covered his patchy hairline. A scarf didn't completely cover a tracheotomy at his neckline. A Marine in full dress guided him gently from behind.

But it was the softness of his voice that highlighted the obvious: his failing health is likely to become a major focus of the president's second term.

In October, the court said the chief justice had begun treatment for thyroid cancer — apparently an aggressive strain. Since then, he has been absent from the bench but has been participating in court business via telephone and messenger from home.

Rehnquist clearly bore the signs of the battle he has been waging, a fight that has included aggressive radiation and chemotherapy treatments. His cheeks were hollow, and there was a white plastic device visible where surgeons performed the tracheotomy Oct. 23 to help him breathe.

Normally, tracheotomy patients can speak only by manually covering the tube that lets air into their throats.

But Rehnquist was able to speak without doing that, suggesting the device he was wearing contained a valve that opens to let air in and then closes automatically so that exhaled air is forced past the vocal chords.

"His ability to walk to the podium, and the way he looked in general, seemed to me encouraging," said Dr. Kenneth Burman, a thyroid specialist at Washington Hospital Center. "But unfortunately, appearance doesn't really tell us anything about the progress of the cancer."

Replacing the chief justice — and perhaps several others on the aging Supreme Court — seems increasingly likely on a court that has served the longest together of any court since the Madison administration.

Rehnquist joined the court in 1972, the last of Richard Nixon's high-court appointments. President Reagan elevated him to Chief Justice in 1986.

For the moment, he appears to be productive, writing two of 16 opinions delivered in the current court term.

At a luncheon after the ceremony, President Bush said, "I want you to know how touched I was that [the] chief justice came to administer the oath." It was "incredibly moving," the president said.

Former senator Alan Simpson, R-Wyo., said he could "tell that he was going to do that, if it took all the energy of his life."

Material from the Los Angeles Times is included in this report.