Washington ? Chief Justice William H. Rehnquist has thyroid cancer, a stunning disclosure Monday that caught even the closest Supreme Court observers off guard and injected into the presidential campaign the issue of appointments to America’s most important legal panel.

Rehnquist’s diagnosis was announced in a terse statement issued by the Supreme Court. It said the 80-year-old widower who has led the court for a generation underwent a tracheotomy over the weekend and was hospitalized but expected to be back at work next week when the court resumes hearing cases.

Left unsaid was Rehnquist’s condition at the National Naval Medical Center in suburban Bethesda, Md., and which type of thyroid cancer he has. About 23,600 people develop various types of thyroid cancer each year in the United States. Most types are considered treatable, but many variables exist, including age and how quickly the cancer is found.

Dr. Yosef Krespi, chairman of otolaryngology at St. Luke’s-Roosevelt Hospital in New York, said only aggressive or complicated thyroid cancers require a tracheotomy. Other physicians said the procedure was sometimes done as part of routine thyroid surgery.

Rehnquist’s hospitalization gave new prominence to a campaign issue that has been overshadowed by the war on terror. The next president probably will name one or more justices to a court that has been deeply divided in recent years on issues as varied as abortion and the 2000 election itself. President Bush won that after the Supreme Court issued a key 5-4 decision in his favor, with Rehnquist as part of the majority.

The last court vacancy was in 1994, the longest stretch of continuity in modern history. Only one of the court’s nine members — Clarence Thomas, appointed by former President Bush — is under 65.

“The Supreme Court has always been in play. This will just increase the salience,” said Nelson Polsby, a political science professor at the University of California, Berkeley, and co-author of “Presidential Elections.”

Rehnquist, a conservative named to the court in 1972 by President Nixon and elevated to chief justice by President Reagan in 1986, has had a series of health problems, including chronic back pain and a 2002 torn leg tendon that required surgery.

He was admitted to the hospital Friday and had the tracheotomy Saturday. During that procedure, a tube is inserted into the patient’s throat, to relieve a breathing obstruction or as preparation for surgery. The court did not explain why it was done on Rehnquist.