We studied MHC class-I and -II phenotypes in adult Caucasian patients with chronic idiopathic autoimmune thrombocytopenia (cAITP). Forty-five patients (median age 51 years, range 21-78 years) with a median disease duration of 7 years (range 2-26 years) were phenotyped for HLA-A, -B, -C by the standard lymphocytotoxicity test, HLA-DR and -DQ by restriction fragment-length polymorphism (RFLP), and -DP by oligonucleotide typing. Antiplatelet antibodies directed against glycoproteins Ib/IX and IIb/IIIa were determined by monoclonal antibody-specific immobilization of platelet antigens (MAIPA). The comparison of antigen frequencies of the whole group of patients with healthy controls revealed no significant difference for any of the MHC class-I or class-II specificities (p > 0.05). Patients were then divided into groups based on (a) their response to therapy, and (b) on whether they did or did not have detectable anti-platelet antibodies (n = 16 versus n = 29). All patients with a poor response to splenectomy carried the HLA-DPB1*0402 phenotype. The HLA-DPB1*1501 allele was found only among patients with detectable antiplatelet antibodies. These differences were not significant after correction for the number of tested antigens, however. Our data suggest that there is no association between MHC class-I/II alleles and adult cAITP or subgroups thereof.