The influence of smoking on duodenal ulcer healing was examined during a double blind study of 83 patients randomly allocated to oxmetidine or cimetidine treatment. Smoking habits were recorded but patients were not advised to change these. Smokers and nonsmokers were similar clinically and did not differ in compliance with medication. Both H2-receptor antagonists were equally effective and after four weeks of treatment ulcers were healed in 76% of patients. Ulcer healing occurred significantly less frequently in smokers (69%) than nonsmokers (89%). Smokers with healed ulcers consumed fewer cigarettes per day (mean +/- SE: 15.8 +/- 1.4) than those whose ulcers did not heal (22.2 +/- 2.7). When smokers were grouped according to daily cigarette consumption, a direct relationship was found between increasing cigarette consumption and decreasing frequency of ulcer healing. No reduction in ulcer healing was apparent in patients who smoked nine cigarettes a day, or less.