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A postmarket surveillance program in outpatients receiving cimetidine was initiated seven months after its approval for marketing. During the first phase of the program, data were obtained over a three-month period for 9,907 patients who received the drug. The overall incidence of adverse effects reported was 4.4%, and the types of adverse effects did not differ from those reported in premarketing controlled studies. Physician response was excellent (85.1%), and the methods used were successful in providing data on a large number of patients who received the drug in routine clinical practice. The results confirmed the safety profile of cimetidine. A follow-up phase, initiated six months after the initial phase of the surveillance program, will provide longer-term data on these patients.