A randomized, prospective, multicenter trial of the effects of cimetidine on benign gastric ulcer was conducted in 60 outpatients. Endoscopic assessment was used as the criterion for healing. Although none of the differences was statistically significant, mean healing rates were higher and mean decreases in ulcer size were greater with cimetidine than they were with placebo. Twenty-four per cent of the ulcers healed completely in 2 weeks when cimetidine was administered, compared with a placebo healing rate of 14 percent. At 6 weeks in the incidence of healing increased to 60 percent in the cimetidine group and 41 percent in the placebo group. The mean percentage of decrease in ulcer size was greater at both 2 and 6 weeks in the cimetidine group than it was in the placebo group. In both, the cimetidine and placebo groups, relatively liberal intake of a potent antacids in treatment of gastric ulcers has not been defined definitively. Thus, a possible beneficial effect of cimetidine may have been obscured. For more clear discimination between the effects of cimetidine and placebo in healing of gastric ulcer, studies utilizing either no antacid or antacids of low neutralizing capacity will be needed.