The phenotype of the major histocompatibility complex, loci A, B, C, DR, and DQ, was studied in 66 patients with Japanese summer-type hypersensitivity pneumonitis induced by Trichosporon cutaneum, and the results were compared with those obtained from 472 normal healthy subjects. Our results showed an increase in the frequency of HLA-DQw3 antigen in the patients (66% compared with 49% in the normal healthy control subjects: χ² = 5.48, p = 0.018, RR = 1.92), suggesting the importance of the host factors for the generation of this disease. The association between HLA-DQw3 as an immune suppression gene and the disease is discussed.