US Democratic Senator Edward Kennedy has a malignant brain tumour, doctors at Massachusetts General Hospital say.
Mr Kennedy, 76, was flown from Cape Cod to the hospital in Boston at the weekend after suffering a seizure.
Doctors says tests reveal a tumour in Mr Kennedy's left parietal lobe. His wife and children have been with him but have not issued a statement.
The youngest brother of assassinated President John F Kennedy, he is one of the best-known Democratic politicians.
"Preliminary results from a biopsy of the brain identified the cause of the seizure as a malignant glioma in the left parietal lobe," doctors Lee Schwamm and Larry Ronan said in a statement.
Mr Kennedy's treatment will be decided after more tests but the usual course includes radiation and chemotherapy, the doctors said.
"He has had no further seizures, remains in good overall condition, and is up and walking around the hospital," they added.
He is expected to remain in hospital for the next few days.
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