Among 56 cases who presented to Kanto-Teishin Hospital complaining of bloody diarrhea or considerable hematochezia of acute onset, 8 cases (14.3%) were considered due to colitis associated with oral ampicillin therapy. The bloody diarrhea, often with abdominal cramps, began 2-7 days after starting the treatment. The dosage of ampicillin taken ranged from 2.0 to 4.5 g. Early total colonoscopy and biopsy revealed marked mucosal hemorrhage with minimal or no inflammatory changes mainly in the right colon. Rectum and sigmoid colon are completely normal except in one case. Symptoms rapidly resolved after the endoscopy. At follow-up colonoscopy, performed 4-12 days later, the mucosal changes had cleared completely. There was no evidence to support a hypersensitivity reaction of the colonic mucosa to ampicillin. We believe that right-sided hemorrhagic colitis is one of the common forms of colitis associated with ampicillin. Its differentiation from other kinds of acute colitis and the importance of early total colonoscopy are discussed.