Stephen Hawking in Pasadena, California in March.
Cambridge University said the 67-year-old is "comfortable" and will stay overnight at Addenbrookes Hospital in Cambridge.
Hawking, one of the world's most famous physicists, is also a cosmologist, astronomer, and mathematician.
Wheelchair-bound Hawking is perhaps most famous for "A Brief History of Time" which explored the origins of the universe in layman's terms, is considered a modern classic.
Hawking has Lou Gehrig's Disease (Amyotrophic Lateral Sclerosis, or ALS), which is usually fatal after three years. Hawking has survived for more than 40 years since his diagnosis.
On his Web site, Hawking has written about living with ALS. "I try to lead as normal a life as possible, and not think about my condition, or regret the things it prevents me from doing, which are not that many," he wrote.
He added: "I have been lucky, that my condition has progressed more slowly than is often the case. But it shows that one need not lose hope."
The disease has left him paralyzed -- he is able to move only a few fingers on one hand. Hawking is completely dependent on others or technology for virtually everything -- bathing, dressing, eating, even speech. He uses a speech synthesizer with an American accent. Watch Larry King's interview with Hawking in 1999 »
Hawking has been married and divorced twice. In 2004, police completed an investigation into accusations by Hawking's daughter that his second wife was abusing him. Authorities said they found no proof.
His Web site says he has three children and one grandchild.
Hawking was born in Oxford, England, on what turned out to be an auspicious date: January 8, 1942 -- the 300th anniversary of the death of astronomer and physicist Galileo Galilei.
A Cambridge University spokesman told CNN: "Professor Hawking is very ill and has been taken by ambulance to Addenbrookes Hospital, Cambridge."
Professor Peter Haynes, head of the university's department of Applied Mathematics and Theoretical Physics, said: "Professor Hawking is a remarkable colleague, we all hope he will be amongst us again soon."
At Cambridge, he holds the position of Lucasian Professor Mathematics -- the prestigious post held from 1669 to 1702 by Sir Isaac Newton.
Hawking has guest-starred, as himself, on Star Trek: The Next Generation and The Simpsons. He also said if he had the choice of meeting Newton or Marilyn Monroe, his choice would be Marilyn.
In October, CNN's Becky Anderson interviewed Hawking. The following are some quotes from that interview:
"Over the last twenty years, observations have to a large extent confirmed the picture I painted in 'A Brief History of Time.' The one major development that was not anticipated was the discovery that the expansion of the universe is accelerating now, rather than slowing down... We live in the most probable of all possible worlds." Watch the CNN interview »
"I believe that the long-term future of the human race must be in space. It will be difficult enough to avoid disaster on planet Earth in the next hundred years, let alone next thousand, or million."
"I see great dangers for the human race ... but I'm an optimist. If we can avoid disaster for the next two centuries, our species should be safe as we spread into space."
CNN's Jennifer Pifer contributed to this report