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A unique outpatient surveillance program consisting of an initial phase (phase I) and a follow-up phase (phase II) was initiated 7 months after FDA approval of cimetidine. The methodology used in this extensive postmarket surveillance program provided information on adverse effects in a large number of patients both on an acute and chronic (up to 12 months) basis. Phase I gathered data on 9907 patients from 1049 physicians over a 3-month period. The overall incidence of adverse effects in this phase was 4.4%, and they did not differ in type from those of premarketing controlled studies. Six months after termination of phase I, phase II was initiated, and follow-up data were requested on the same group of patients from the same 1049 physicians. Case report forms were received from 905 physicians on 7248 patients representing 9763 courses of cimetidine therapy. A total of 162 adverse effects were reported by 138 patients for an incidence of 1.8%. As in postmarket surveillance I, the adverse effects reported in postmarket surveillance II were not different from those previously reported in clinical studies, published reports, or via the spontaneous reporting system. This postmarket surveillance program designed and successfully tested methodology useful for future surveillance studies, and confirmed the safety of cimetidine in a large cohort of patients which remains a valuable source for future data accumulation.