Prof Hawking has cancelled an appearance in the US
Leading scientist Stephen Hawking's condition has "improved" after being admitted to hospital with chest problems, Cambridge University says.
His employers said Professor Hawking was undergoing tests on Monday at Addenbrooke's Hospital, Cambridge.
A university spokesman said the 67-year-old, who has motor neurone disease, was now "comfortable".
Prof Hawking, author of A Brief History of Time, has worked at Cambridge University for more than 30 years.
The world-famous physicist, who uses a wheelchair and speaks with the aid of a voice synthesiser, had flown to the US at the end of February to the California Institute of Technology where he is a visiting scholar.
But he had not been well for some weeks and called off an appearance at Arizona State University on 6 April because of his illness.
He was flown back to the UK on Saturday and was admitted to Addenbrooke's on Monday after being seen by a doctor.
Prof Peter Haynes, head of the University's Department of Applied Mathematics and Theoretical Physics, where Prof Hawking works, said: "Professor Hawking is a remarkable colleague.
"We all hope he will be amongst us again soon."
Prof Hawking developed symptoms of motor neurone disease while studying in the 1960s and is one of the world's longest-surviving sufferers.
The scientist has three children and one grandchild. He became a CBE in 1982 and a Companion of Honour in 1989.
Last year, it was announced he would be stepping down as the university's Lucasian Professor of Mathematics at the end of this academic year.
It is policy for holders of the title to retire at 67.
However, Prof Hawking said he intended to continue working as Emeritus Lucasian Professor of Mathematics.