A 30-year-old male admitted to out hospital complaing of petechiae. He had attack of fever 2 days before admission, without parotitis and orchitis. Laboratory data showed marked thrombocytopenia. Bone marrow showed normocellularity with an increase of megakaryocytes. Antimumps IgM was positive by the EIA. He was diagnosed as idiopathic thrombocytopenic purpura (ITP) associated with mumps virus infection. After the administration of oral prednisolone and 5 days-infusion of gamma-globulin, platelet count increased rapidly. Prednisolone discontinued within 35 days, as in acute ITP, and he maintained remission. In conclusion, the test for antiviral antibodies is indispensable to exclude acute ITP in adult ITP patients.