A series of 31 infants and children with acute duodenal ulcer verified by endoscopy was studied over an eight year period. Eighteen (58%) of them were under 2 years of age. The most common symptom was upper gastrointestinal bleeding (n = 27, 87%). Twenty nine patients (94%) had a preceding illness characterised by diarrhoea, upper respiratory tract infection, or fever, which was not necessarily treated with antipyretic drugs. Initial endoscopy showed that ulcer lesions were solitary in 14 patients and present on the anterior wall (n = 11), posterior wall (n = 2), or both (n = 1). Multiple ulcers were found in 17 patients, and present in the bulb with (n = 6) or without (n = 11) extension into the second part of duodenum. The most conspicuous finding was the irregularly shaped ulcers seen in eight young children with similar clinical and endoscopic features. Sixteen patients were re-endoscoped one to two weeks after the initial examination; the ulcers had entirely disappeared in 13, and there were only small residual ulcers in three. Thirty patients were treated medically and only one (with uncontrollable haemorrhage) required operation. Most patients were symptom free two to six years after the initial diagnosis. Our results suggest that young children may develop acute duodenal ulcers after viral illnesses whether or not they are treated with drugs, mainly antipyretics. This kind of acute duodenal ulcer usually heals quickly irrespective of the morphology, site, and number of ulcers.