Recent reports of cimetidine toxicity are summarized. Summaries of specific cases and categorized according to cardiovascular, central nervous system, dermatologic, endocrine, gastrointestinal, hematologic, or renal toxicity, or overdosage. Adverse reactions reported secondary to cimetidine during its investigational period and shortly after marketing were minimal. In several studies in which over 1200 patients were treated with cimetidine, the incidences of adverse clinical symptoms was no higher than in the nearly 500 placebo-treated patients. However, subsequent reports indicate that elderly patients, patients with impaired renal function, and patients with liver disease appear quite susceptible to mental confusion. Potentially serious hematologic depression, cardiac depression, and hypersensitivity-type hepatitis have also been reported. As a result of the reports of impotence and oligospermia, controlled trials evaluating the effect of cimetidine on fertility in young men are needed.