A 55-year-old woman was admitted with a cough and fever in August. A diagnosis of Japanese summer-type hypersensitivity pneumonitis was made on the basis of radiological, serological and pathological findings, in addition to positive returning home provocation. Serum KL-6 was monitored during the clinical course. Although KL-6 fluctuated slowly in comparison with the clinical symptoms and HRCT findings, it was considered useful for confirming the effects of treatment. Serum anti-Trichosporon antibody and the phenotype of HLA were studied in both the patient and her asymptomatic roommate, with whom she had no blood relationship. Though both were sensitized immunologically, HLA-DQ 3, which was reported to be associated with Japanese summer-type hypersensitivity pneumonitis, was detected in the patient but not in her roommate. It was suggested that HLA plays a role in the development of this disease.