Chief Justice William H. Rehnquist (search) will be absent from the Supreme Court (search) on Monday, beginning a sixth week of missed work caused by thyroid cancer.

The 80-year-old has been receiving chemotherapy (search) and radiation treatment at home and plans to skip the two-week cycle of arguments that starts Monday, a court spokesman said Friday. No other information was available.

Details have been kept secret about the seriousness of Rehnquist's cancer, which was announced in a terse statement Oct. 25. Some doctors familiar with thyroid cancer (search) say his treatment suggests a fast-growing cancer.

Although Rehnquist is working from his home in suburban Virginia, the chief justice's absence is fueling intense speculation that he will step down soon and give the court its first opening since 1994.

However troubling Rehnquist's inability to attend the last two-week session of arguments before the holiday recess, the most telling sign will be if he misses the Jan. 20 presidential inauguration. The chief justice traditionally swears in a new president.

"I think the key date is after the turn of the year," said Douglas Kmiec, a Pepperdine University law professor and former legal counsel to President Reagan and the first President Bush. "If he didn't believe he was able to keep up his work in full at that time, he would take himself out."

Rehnquist has been on the court since 1972 and chief justice since 1986. He has had other health problems including chronic back pain and a torn leg tendon that required surgery. Besides Rehnquist, three other justices have had cancer. John Paul Stevens (search) had prostate cancer, Sandra Day O'Connor had breast cancer, and Ruth Bader Ginsburg was treated for colon cancer.

Since his illness was announced, Rehnquist has ruled on cases by reviewing transcripts of arguments and passing along his votes to his fellow justices. Opinions are largely researched and drafted by law clerks, and administrative tasks fall to a top aide.

The court will recess Dec. 14 for the holidays and return for arguments Jan. 10.