Mr Clinton, 75, is "on the mend" and "in good spirits" after being admitted to UC Irvine Medical Center on Tuesday, spokesman Angel Ureña said.
Though it is unclear what led to the admission, Mr Clinton has a history of heart disease.
A statement from his doctors said he had received antibiotics and fluids.
The physicians added that Mr Clinton remained at the hospital for "continuous monitoring".
"After two days of treatment, his white blood cell count is trending down and he is responding to antibiotics well."
The doctors at UC Irvine are in contact with Mr Clinton's medical team in New York, as well as his cardiologist, and said they thought he would be released home soon.
Mr Clinton served as US president from 1993 until 2001. He was impeached in 1998 for lying to investigators about his affair with White House intern Monica Lewinsky. He was acquitted at his Senate trial.
In 2004, Mr Clinton had a quadruple bypass surgery and ten years later had a clogged artery opened after complaining of chest pains.
Not long after the second surgery, the former president, who was known for his love of fatty foods, became vegan.