The reason why causes idiopathic thrombocytopenic purpura(ITP) was usually unknown. However, we clinically experience these cases after some viral infections. Viral infection can induce the autoimmune diseases including ITP. Probably most of acute and/or chronic ITP may be immunologically caused by molecular mimicry, and antibodies formed in response to viral infection may cross-react with antigenic targets naturally present on platelet. We explain the two reports on the chronic ITP following viral infections of influenza A and hepatitis C in detail.