A single-blind study of 339 patients in 19 centres compared the efficacy and tolerance of ranitidine in treating endoscopically confirmed gastric ulcers. Ranitidine (150 mg twice daily) was compared with cimetidine (1 g daily in divided doses) over 4 weeks, followed by a second 4-week treatment for any patient whose ulcer was not healed. In 292 patients who completed the study, endoscopy showed healing in 69% of patients receiving ranitidine and 59% receiving cimetidine after 4 weeks, and 90% and 88%, respectively, by 8 weeks. These results were not significantly different, and, similarly, healing rates for different ulcer sites did not differ. There were no serious adverse drug reactions during the study. Ranitidine is an effective and safe treatment for healing gastric ulcers, with a tendency to produce a faster healing rate than cimetidine during the first 4 weeks of treatment.