A 29-year old woman who was on steroids for Behcet's disease received intravenous cimetidine for severe gastrointestinal bleeding. A bone-marrow examination performed a few days prior to cimetidine treatment showed a normally active marrow. On the fifth day of cimetidine therapy, agranulocytosis (800/ml) and a very low platelet count (40,000/ml) were observed; bone-marrow failure was documented both by biopsy and aspiration. No other drug known to be myelotoxic was given. This severe hematologic side effect of cimetidine resolved within 10 days after discontinuation of the drug.