Recent studies have noted an increased association of the histocompatibility antigen HLA-B8 with myasthenia gravis in Caucasian patients. The HLA types of 63 Japanese patients with myasthenia were compared to those of 271 controls. The present study was designed to assess correlations between HLA antigens, sex, age at onset, the presence of autoantibodies and thymic morphology. HLA-B12 was increased Japanese patients, especially females with early age at onset, and in addition it was significantly correlated with thymic hyperplasia. HLA-B5 was frequently found in the patients with thymoma. Statistically the most frequent haplotype found in this disease was HLA-A10-HLA-B12.