Byrd has been in Congress since 1952 and was first elected to the Senate six years later.
He has been plagued by health problems in recent years, uses a wheelchair and sometimes appears frail. He has been hospitalized several times.
He stepped down as chairman of the powerful Senate Appropriations Committee last November, but not before first chastising party leaders whom he said had been discussing his removal.
In the statement, Byrd's office said the senator was hospitalized late Friday afternoon as a "precautionary measure." It said his doctors attributed the temperature to a minor infection.
The statement did not say why no word was released for more than 48 hours after the senator was taken to the hospital, nor did it identify the facility where he is being treated.
Slideshow archive | ABC13 wireless | Help solve crimes