BOSTON — A cancerous brain tumor caused the seizure Sen. Edward M. Kennedy suffered over the weekend, doctors said Tuesday in a grim diagnosis for one of American politics' most enduring figures.

"He remains in good spirits and full of energy," the doctors for the 76-year-old Massachusetts Democrat said in a statement.

They said tests conducted after the seizure showed a tumor in Kennedy's left parietal lobe. Preliminary results from a biopsy of the brain identified the cause of the seizure as a malignant glioma, they said.

His treatment will be decided after more tests but the usual course includes combinations of radiation and chemotherapy.

Kennedy has been hospitalized in Boston since Saturday, when he was airlifted from Cape Cod after a seizure at his home.

"He has had no further seizures, remains in good overall condition, and is up and walking around the hospital," said the statement by Dr. Lee Schwamm, vice chairman of the Department of Neurology at Massachusetts General Hospital and Dr. Larry Ronan, Kennedy's primary care physician.

They said Kennedy will remain in the hospital "for the next couple of days according to routine protocol."

Kennedy's wife and children have been with him each day since he was hospitalized. Senator Kennedy's son, Rep. Patrick Kennedy, D-R.I., plans to stay at the hospital for the time being.

"Obviously it's tough news for any son to hear," said spokeswoman Robin Costello. "He's comforted by the fact that his dad is such a fighter, and if anyone can get through something as challenging as this, it would be his father. So he's optimistic, he's hopeful, but obviously he's concerned."

President Bush was notified by his staff of Kennedy's diagnosis at 1:20 p.m.

"He said he was deeply saddened and would keep Senator Kennedy in his prayers," spokeswoman Dana Perino said.

Malignant gliomas are a type of brain cancer diagnosed in about 9,000 Americans a year — and the most common type among adults. It's an initial diagnosis: How well patients fare depends on what specific tumor type is determined by further testing.

Average survival can range from less than a year for very advanced and aggressive types — such as glioblastomas — or to about five years for different types that are slower growing.

News of the diagnosis hit hard for colleagues on both sides of the aisle.

"I'm really sad," former Sen. Bob Kerrey, D-Neb., said when told in a Senate hallway about Kennedy's condition. "He's the one politician who brings tears to my eyes when he speaks."

"I am so deeply saddened I have lost the words," Sen. John Warner, R-Va., said in a Senate hallway. Warner said he and Kennedy had been friends for 40 years. Both served on the Senate Armed Services Committee together.