Two hundred and seventeen gastric ulcer patients whose ulcers had healed during omeprazole (30 mg) or cimetidine (1 g) treatment entered a follow-up study for 6 months. Endoscopy was performed after 6 months, or earlier if the patient had a symptomatic relapse. Both the initial treatment and the follow-up study were double-blind. Ulcers recurred in 35% of the omeprazole-treated patients and in 41% of the cimetidine-treated patients (p > 0.05; log-rank test). The recurrence rate was 45% among smokers, compared with 25% among non-smokers (p < 0.05). Ulcers also recurred more rapidly (p < 0.05) in patients with a healed prepyloric ulcer (49%) than in patients with a healed corpus ulcer (23%).