Transient thrombocytopenia was noted to occur in ten of 13 patients with advanced carcinoma of the prostate undergoing intravenous diethylstilbestrol diphosphate therapy. This was not associated with abnormalities of the initial platelet count or other parameters of the hemogram. There was no evidence of disseminated intravascular coagulation in these patients. No serious bleeding episodes occurred secondary to the thrombocytopenia. Following diethylstilbestrol diphosphate infusion, platelet counts generally returned to pretreatment levels. While the mechanism of action of the thrombocytopenia is unclear, we hypothesize a transient bone marrow toxicity as the cause of this phenomenon.