Acute thrombocytopenia due to intravenous infusion of radiographic contrast media is extremely rare. We report the first such case in Japan. A 52-year-old male was admitted to hospital because of right ureteral stone. Two hours after drip infusion pyelography, the patient experienced chills and fever. It was shown that his platelet count had decreased abruptly from 233,000 to 8000/mm3. Platelet associated immunoglobulin was not detected. An in vitro test for platelet aggregation by contrast media was negative. Following infusion of hydrocortisone, the platelet count recovered to a normal level within several days.