We report a patient with idiopathic thrombocytopenic purpura (ITP) in remission, who relapsed as a result of an influenza A virus infection. A 41-year-old woman presented with fever elevation, coughing, and generalized petechiae. Her platelet count had decreased to 1 x 10(9)/l. She had been diagnosed with ITP at age 23, and continuous complete remission had followed steroid therapy and splenectomy. Influenza A antigen was positive in her pharyngeal aspirate, and oseltamivir was effective for her symptoms. Findings of a bone marrow smear were typical for ITP. Steroid therapy resulted in a second complete remission. Although the development of ITP caused by influenza infection and a relapse caused by an influenza vaccination have been previously described, a relapse caused by a sporadic infection has never been documented to our knowledge. Physicians should carefully monitor the hematological data of influenza patients, especially those with ITP, even in remission.