US Senator Edward Kennedy has been diagnosed with a malignant brain tumour.
Mr Kennedy will require chemotherapy and radiation therapy.
The 76-year-old has been in the Massachusetts General Hospital since he had a seizure on Saturday last.
Doctors said a biopsy had identified a malignant glioma in his left parietal lobe.
The Taoiseach has said the thoughts and sympathies of the Irish people are with Senator Kennedy and his family.
US President George W Bush said he was 'concerned' for the veteran Senator, who has been a fierce critic of his administration.
A Government spokesperson said the Irish Ambassador to Washington, Michael Collins, had been in contact with the Kennedys since the Senator's admission to hospital to express the Government's concern.
The US National Cancer Institute says the outlook for tumour patients with such a diagnosis is poor, with average life expectancy from a few months to up to five years.
