A 73-year-old woman was admitted to our hospital with a low-grade fever, dry cough and dyspnea on exertion as the chief complaints. She had been a professional shiitake mushroom grower for 50 years. Three years before visiting our hospital, she had been suspected of having hypersensitivity pneumonitis as a result of chest X-ray examination, bronchoalveolar lavage and transbronchial lung biopsy performed at another clinic. No antigens were identified at that time, but prednisolone was administered. On admission to our hospital, chest radiography and chest computed tomography revealed an interstitial shadow with subpleural honey-combing in both lower lung fields. After steroid pulse therapy, dyspnea on exertion and hypoxia improved moderately. Because of recurrence of the dyspnea, however, she was admitted on four separate occasions. On the second admission, an increase in lymphocytes was found by bronchoalveolar lavage, and septal lymphocytic infiltration accompanying fibrosis was demonstrated by transbronchial lung biopsy. On the fourth admission, a detailed immunological examination and an environmental survey were performed. The environmental provocation test yielded clinical symptoms similar to those experienced at the mushroom farm. Furthermore, tests of precipitation and lymphocyte proliferation in response to shiitake mushroom extracts were positive. Finally a diagnosis of chronic hypersensitivity pneumonitis induced by shiitake mushrooms was confirmed.