Sixty-six patients, diagnosed as Japanese summer-type hypersensitivity pneumonitis at Osaka Prefectural Habikino hospital between 1973 and 1980, were studied. The diagnosis was based on the clinical features and summer-seasonal nature of the disease. The presence of an aetiological agent within patients' home environment was suggested by the recurrence of acute symptoms of high fever, cough and dyspnoea 5–8 hr after coming home from hospital, and by spontaneous improvement on leaving home. Immunological studies revealed the presence of anti-Cryptococcus antibody in sixty-four of sixty-five patients' sera, by indirect immunofluorescence against Cryptococcus neoformans, Precipitating antibody against culture supernatant protein-antigen of Cr. neoformans was detected in more than 80% of sera obtained from patients during the active stage of the disease. The positive result on inhalation provocation-challenge, using culture supernatant protein-antigen, suggested that Cr. neoformans or antigenically related Cryptococcus species may cause Japanese summer-type hypersensitivity pneumonitis.