50 consecutive patients with radiographically verified duodenal ulcer and a history of peptic ulcer disease for more than 3 years and with elevated gastric acid secretion in the basal state and after stimulation with pentagastrin were randomly allocated to two groups. The first group of 25 patients was treated with benzilonium bromide (Ulcoban Prolongatum, Parke, Davis & Co.) and the second group was subjected to selective proximal vagotomy (SPV). The patient's gastric acid secretion was determined before the start of treatment and after 1, 6, and 12 months. After one year the basal acid output had decreased by 58.2% in the group treated with benzilonium bromide and by 82.9% in the patients who had undergone SPV. The peak acid output fell by 49.4% in the benzilonium group and by 66.1% in the SPV group.