The value of cimetidine in treatment of duodenal ulcer and the Zollinger-Ellison syndrome appears to be well established. The drug has been enthusiastically embraced and widely used by practicing physicians. As with virtually all drugs used in the practice of medicine, cimetidine is not without its adverse effects. In some instances these effects may result from actions of cimetidine on H2-receptors on many widely distributed and diverse cells other than parietal cells, to which its potent acid-inhibiting properties are directed. Other adverse effects of cimetidine may be idiosyncratic, and, therefore, not predictable on a pharmacologic basis. In some instances the mechanisms responsible for cimetidine's adverse effects hav e yet to be defined. An assortment of abnormalities reported in patients receiving cimetidine have been suggested, but not proven, to represent adverse effects of the drug. Considering its extremely wide use, serious toxicity with cimetidine is rare. However, no potent drug, including cimetidine, used in the practice of medicine is without its adverse effects. Recognizing the present and projected extensive and probably long-term use of cimetidine, physicians and surgeons treating patients with cimetidine must maintain continued surveillance in order to detect and clarify potential undesired consequences of cimetidine administration.