Tobacco smoking delays healing of gastric ulcer and may influence duodenal ulceration. Seventy men, all cigarette smokers, were found to have duodenal ulceration at endoscopy. All were advised to stop smoking and received a three-month course of cimetidine. Endoscopy was repeated at three months (n = 63) and at six months (n = 56). At three months most (79%) patients showed ulcer healing and there was no difference between men who had and had not stopped smoking. At six months, however, a higher proportion (61% vs 28%, p less than 0.05) of smokers (n = 38) than ex-smokers (n = 18) had duodenal ulceration. This difference reflected a combination of increased ulcer persistence and ulcer relapse. Neither cimetidine nor cigarette smoking nor ulcer healing appeared substantially to affect duodenitis and fixed deformity. We conclude that continued cigarette smoking does not prevent the powerful duodenal ulcer healing effect of cimetidine but does predispose to an increased expectation of duodenal ulceration soon after cimetidine has been stopped.