The efficacy and safety of aluminium phosphate and ranitidine in the short-term treatment of duodenal ulcer were compared in a multicenter, randomized, double-blind study. A total of 304 patients with endoscopically proven active duodenal ulcer were recruited: 153 received 11 g of aluminium phosphate gel five times a day (equivalent to a daily acid buffering capacity of 182 mEq/HC1) and 151 rantitidine 300 mg once daily for 6 weeks. At the end of the treatment period, 74 out of 113 patients (65%) treated with aluminium phosphate and 84 out of 105 patients (80%) treated with rantitidine showed ulcer healing at endoscopy (p = 0.02). Ulcer symptoms, identified as frequency and intensity of daytime and nocturnal epigastric pain, were significantly reduced by both treatments, but rantitidine proved more effective than aluminium phosphate in reducing the frequency of daytime pain (p < 0.01) and its severity (p < 0.01); in contrast, no significant differences were found with regard to frequency and severity of nocturnal pain. The incidence of unwanted effects was significantly higher in the aluminium phosphate group. The main adverse event observed was constipation which, however, was hardly ever severe enough to warrant discontinuing treatment.