The safety and efficacy of the new, highly potent histamine H2 receptor antagonist, famotidine, was evaluated in nine patients with Zollinger-Ellison syndrome. Famotidine was found to be safe and effective for long-term therapy of Zollinger-Ellison syndrome, and its onset of action was similar to those of equipotent doses of cimetidine and ranitidine. However, in these patients famotidine was approximately nine times more potent than ranitidine and approximately 32 times more potent than cimetidine, furthermore its duration of action was significantly longer than that of these two drugs. The authors conclude that famotidine promises to be the histamine H2 receptor antagonist of choice for treating the Zollinger-Ellison syndrome.