The effect of treatment for 4 weeks with the H2-receptor antagonist ranitidine 200 mg daily on ulcer healing, clinical symptoms and antacid consumption, and on gastric acid secretion, was studied in a double blind trial in 48 patients with a total of 50 endoscopically confirmed duodenal, prepyloric or corporeal gastric ulcer. Patients whose ulcers did not show complete healing within 28 days were continued openly on ranitidine for up to a further 4 weeks. Endoscopy, basal gastric acid secretion (BAO) and pentagastrin-stimulated maximal secretion (PAO) studies were performed at 2-week intervals. After four weeks, 73% of the gastro-duodenal ulcers in the ranitidine group showed complete healing versus 42% in the placebo group (p less than 0.05). Gastric acid secretion was considerably inhibited both under basal (89%; p less than 0.001) and maximal challenge (71%; p less than 0.001) conditions. The inhibitory effect was still pronounced 13-15 h after administration of ranitidine 100 mg. Symptoms and the need for antacids were significantly reduced. Ranitidine appears to be an efficacious, safe and well tolerated medicine principle for the treatment of gastro-duodenal ulcer disease.