Although both primary biliary cirrhosis (PBC) and idiopathic thrombocytopenic purpura (ITP) are autoimmune diseases, the association of the 2 diseases is rare. Here, we report a case of ITP that developed during the follow-up of PBC in a 74-year-old man. The patient had been diagnosed with PBC 12 years previously, and had received treatment with ursodeoxycholic acid. The platelet count decreased from approximately 60 x 10(9)/L to 8 x 10(9)/L, and the association of decompensated liver cirrhosis (PBC) with ITP was diagnosed. Steroid and immune gamma globulin therapy were successful in increasing the platelet count. Interestingly, human leukocyte antigen genotyping detected the alleles DQB1*0601 and DRB1*0803, which are related to both PBC and ITP in Japanese patients. This case suggests common immunogenetic factors might be involved in the development of PBC and ITP.