In a study of 28 outpatients with endoscopically proven duodenal ulcers, 14 patients (with a total of 15 ulcers) were treated with bismuth tablets (colloidal bismuth subcitrate, De-Nol, Gist-Brocades NV) and 14 patients (14 ulcers) were treated with cimetidine (Smith, Kline, and French). Clinical and endoscopic assessments were made after four and six weeks' therapy. After four weeks, 10 of the bismuth treated ulcers (67%) and eight of the cimetidine treated ulcers (57%) were completely healed. After six weeks of therapy, complete healing was seen in 86% of both the bismuth treated and the cimetidine treated ulcers. Twenty of the 24 completely healed ulcer patients (10 of each group) cooperated in a three month follow-up study. Pain recurred in three patients of the bismuth group and four of the cimetidine group and they were examined endoscopically. A recurrent ulcer was found in one of the bismuth treated patients and in three of the cimetidine treated patients. These observations indicate that colloidal bismuth subcitrate was at least as effective as cimetidine in the healing of duodenal ulcer.