Hematopoietic function of some aplastic anemia (AA) patients is dependent on the administration of cyclosporine (CyA). To investigate whether certain HLA class II genes are associated with susceptibility to such CyA-dependent AA, we determined the HLA class II alleles of 59 AA patients treated with CyA. Among 26 patients successfully treated with CyA, 13 required a small dose of CyA to maintain stable hematopoiesis. Of these 13 AA patients, 10 shared an HLA class II haplotype of DRB1*1501-DQA1*0102-DQB1*0602. None of the 13 responders who obtained a sustained remission off CyA therapy possessed this haplotype. In the 10 patients who shared the HLA class II haplotype, single-strand conformation polymorphism analysis of each gene fragment of this haplotype failed to detect a polymorphism in the nucleotide sequence. When the AA patients were assessed for their likelihood to respond to CyA therapy, the response rate in patients with this haplotype (71%) was significantly higher than that of patients with another haplotype associated with HLA-DR2, DRB1*1502-DQA1*0103- DQB1*0601 (36%) and that of patients without HLA-DR2 (35%). These findings indicate that the CyA-dependent response of AA is closely related to an HLA class II haplotype of DRB1*1501-DQA1*0102-DQB1*0602 and suggest that, in AA patients with this haplotype, immune mechanisms play an important role in the pathogenesis of bone marrow failure.