Summer-type hypersensitivity pneumonitis(SHP) first reported by T. OCHI, et al. (1978), as a new type hypersensitivity pneumonitis(HP) with features of initiation during summer and anti-Cryptococcus antibody positive sera, has been recognized as "a unique disease in Japan", a most common type of HP in Japan, and now also known as anti-Trichosporon cutaneum antibody-positive SHP. This report was mainly concerned with the histopathology of SHP, thus far diagnosed in our hospital. Of the cases in our hospital, 62 consecutive biopsied cases (3 cases of open lung biopsy and 59 cases of transbronchial lung biopsy) without steroid institution before lung biopsy have been reviewed and revealed granulomatous interstitial pneumonitis in the bronchiolo-alveolar region, like various types of HP. Alveolitis (61 of 62 cases; 98.4%), sarcoid-like granuloma (50 of 62; 80.6%) and Masson's body (36 of 62; 58.1%) are main features, and could be named "triad" features. Concerning histopathological findings related with the period of lung biopsy after onset of clinical episodes as HP, alveolitis has been present regardless of period, and fibrinous exudate present in the early period and not in a case at 4 months. Sarcoid-like granuloma and Masson's body have appeared firstly at one-month cases. After this period sarcoid-like granuloma have been present in nearly all cases, but Masson's body has been not present in 10-month case with gradual decrease after 2 months, when all 2-month cases had it. In summary, for histopathological diagnosis of HP, the "triad" features and the time of lung biopsy done are keys, taking into consideration histopathological differential diagnosis.(ABSTRACT TRUNCATED AT 250 WORDS)