A 70-year-old man was admitted to our hospital because of right hemiparesis. He had no allergies or previous exposure to radiographic contrast medium, and the platelet count on admission was within the normal range. On day 8 of hospitalization, he underwent computed tomography of the brain with 100 ml of iopamidol administered intravenously. Three hours later, his platelet count fell to 5,000/microliter, and he developed purpura. Because drug-induced thrombocytopenia was suspected, platelet transfusion was undertaken and corticosteroids were administered. The platelet count returned gradually to normal in 2 days. At the time, we were unable to ascertain the cause of the thrombocytopenia. To clarify whether the contrast medium had been responsible, iopamidol was added to the patient's heparinized whole blood. Subsequent platelet aggregation was observed microscopically and the platelet count decreased, suggesting that the thrombocytopenia had been due to contrast medium-induced platelet aggregation. Although thrombocytopenia after injection of contrast medium is extremely rare, such cases should be evaluated carefully because the condition can be life-threatening if severe.