Ashcroft, a former member of the Senate who has been free of health problems, canceled an appearance Thursday afternoon at which he was to have announced verdicts in a terrorism case.
Justice Department spokesman Mark Corallo, said the 61-year-old attorney general initially believed he had stomach flu.
"He went home and when the condition worsened, he was visited by White House Physician Daniel Parks, M.D. who advised that he go to the emergency room," Corallo said.
He said Ashcroft was taken Thursday night to the emergency room of the George Washington University Hospital here for evaluation of stomach complaints.
"After a full medical work up in the emergency room," Corallo said, "it was determined that he was suffering from a severe case of gallstone pancreatitis. He was admitted to intensive care for careful monitoring and is being treated with antibiotics."
The spokesman added, "Doctors expect to know more within 48 hours."
Corallo said he expects Ashcroft will spend a couple of days in the hospital.
According to the National Institutes of health, pancreatitis is an inflammation of the pancreas, a big gland behind the stomach. The Pancreas releases insulin to help the body use sugars that come from food, as well as enzymes that digest foods.
Normally, the enzymes do not begin to work until they leave the pancreas. Pancreatitis occurs when they begin to work inside the organ, eating away at it.
Gallstones form when bile, a digestive liquid, hardens into solid pieces. If they get lodged in the ducts leading out of the pancreas, enzymes can get backed up and pancreatitis occurs.
Ashcroft served one term as a U.S. senator from Missouri before losing in 2000 to Mel Carnahan, who had died in a place crash shortly before the election. Carnahan's wife Jean had arranged to take the seat if her late husband won.
Following the defeat, President Bush picked Ashcroft to be his attorney general. The conservative Ashcroft was widely considered the most controversial of the president's early personnel decisions, and at his confirmation hearings, Ashcroft's positions on race and religion were scrutinized.
Since Sept. 11, Ashcroft has been a leading advocate for increasing the power of law enforcement and a chief defender of the controversial USA PATRIOT act passed in the days after the 2002 attacks.
Prior to his election to the Senate, Ashcroft served as Missouri's governor from 1985 to 1993 and was state attorney general for two terms. He is a graduate of Yale University and the University of Chicago law school. He and his wife Janet have three children and a grandchild, according to a biography on the Justice Department Web site.
Under the 25th amendment, Ashcroft is seventh in line in presidential succession, behind the vice president, the speaker of the House of Representative, the Senate's president pro tempore, and the secretaries of state, defense and treasury.