ASHINGTON, Jan. 20 - Whether Chief Justice William H. Rehnquist's participation in the inauguration signified a last hurrah or a re-emergence after three months of intensive treatment for thyroid cancer was, on the basis of his 13 minutes on the platform, impossible to know.

For the moment, what counted was that he was there as he had promised to be, walking down the stairs to the platform under his own power, though with a cane, and administering the oath of office to President Bush in a firm if somewhat husky and unfamiliar-sounding voice. The chief justice had a tracheotomy, to assist him in breathing, on Oct. 22, several days after he last appeared in public on the Supreme Court's bench.

Since last month, the chief justice, who marked his 33rd anniversary on the court on Jan. 7, has been working in his chambers there and has attended the justices' weekly closed-door conferences. But he has not presided over the court's public argument sessions, leading to widespread speculation about the gravity of his condition and the imminence of his retirement.

What the public saw on Thursday was an 80-year-old man who was obviously not in robust health. But his appearance has not changed drastically during his months of illness and treatment, in which he has been receiving both radiation and chemotherapy.

Misidentified twice as "the chief justice of the United States Supreme Court" by Senator Trent Lott, who was announcing the names of the platform guests - the title is "chief justice of the United States" - Chief Justice Rehnquist was the last of the dignitaries to arrive. The eight other justices were already seated, several having attended the traditional inauguration-morning breakfast at the court with family members and guests. The chief justice came directly from his home in suburban Arlington, Va., and was accompanied to the platform by Sally Rider, his administrative assistant.

There was a slight smile on his face as he walked down the steps to his seat, whether from embarrassment at suddenly being the center of attention or from satisfaction at having accomplished a goal that some had speculated would be out of reach.

After he administered the oath to President Bush, he added "congratulations," a coda different from the ambiguous "good luck" he offered to Bill Clinton on Jan. 20, 1997, a week after the Supreme Court heard arguments in the Paula Jones sexual harassment case against that re-elected president.

Once he had administered the oath on Thursday, the chief justice left the platform almost immediately. He did not stay to hear Mr. Bush's Inaugural Address and did not attend the lunch that followed at the Capitol, in the Statuary Hall. At the lunch, the president said he had been "touched" by the chief justice's presence and found it an "incredibly moving" part of the ceremony.