The aim of the study was to determine the effectiveness of cimetidine in a dose of 400 mg twice daily in the prevention of gastric ulcer recurrence over a 2-yr period. The trial was double-blind and placebo-controlled; 24 patients received cimetidine and 25 received placebo. All had had a gastric ulcer within the prior 6 wk, and healing was demonstrated by endoscopy or by barium meal. The patients were contacted at monthly intervals, and if symptoms were present, investigations were performed. Annual barium meal examinations or endoscopy were also performed in all except 7 patients. In the placebo group, there was rapid recurrence within the first 12 mo; 12 of the 13 recurrences that occurred in this group over the 2-yr period took place within the first 12 mo. In the cimetidine-treated group, there were eight recurrences over the 2-yr period, five of which occurred in the first year. Log rank comparison showed a significant benefit due to cimetidine at 1 yr, but not over the 2-yr period.