In a randomized double blind multicenter trial, patients treated with cimetidine (800 or 1200 mg daily) or an intensive regimen of Al-Mg antacid (210 ml daily) had similar rates of duodenal ulcer healing and pain relief. After 4 weeks of treatment, the proportion of patients with ulcer healing by endoscopy were: cimetidine (1200 mg), 21 of 33 (64 percent); cimetidine (800 mg), 19 of 32 (59 percent); and antacids, 15 of 29 (52 percent). These proportions did not differ significantly. Eighty per cent of cimetidine-treated patients became asymptomatic by week 4, as did 63 percent of antacid-treated patients (P greater than 0.1). No untoward effects were observed during cimetidine treatment. Twenty-seven per cent of antacid-treated patients reported diarrhea.