Doctors and people close to Senator Kennedy said that he would remain in the hospital for the next couple of days. The doctors said he was “in overall good condition” and “remains in good spirits and full of energy.” He has not had another seizure since he was hospitalized, they said.

“Right now, he’s his normal self, except for the news that he’s dealing with,” said a close friend who spoke on the condition of anonymity. “I bet he’ll be back at the Cape sailing this weekend. I expect he’ll go back to work” after the Memorial Day recess.

Senate Democrats and Republicans learned of Mr. Kennedy’s condition as they were gathered for their weekly closed-door party luncheons, and members of both parties were visibly shaken by the news.

As he opened debate on the Iraq spending bill, Senator Robert C. Byrd of West Virginia, at 90 the only current senator to serve longer than Mr. Kennedy, was distraught. “Ted, Ted, my dear friend, I love you and miss you,” Mr. Byrd said in halting remarks on the floor.

Malignant glioma is the most common form of brain cancer, accounting for about 9,000 cases diagnosed each year in the United States, according to the National Cancer Institute. They are more common in older people, especially those between the ages of 75 and 84, according to the American Cancer Society.

The prognosis varies depending on the type and severity of the tumor, and the patient’s age. The American Cancer Society said that survival rates drop with increasing age.

Image Senator Edward M. Kennedy with Senator Barack Obama at a campaign rally at American University in Washington on Jan. 28. Credit... Brendan Smialowski for The New York Times

Dr. Patrick Y. Wen, clinical director of the Center for Neuro-Oncology at the Dana-Farber Cancer Institute, said the average prognosis for the most aggressive type of glioma is 14 to 15 months, while the prognosis for slower-growing tumors is two to four years.