A 52-year-old man was hospitalized because of recurrent fever, dyspnea and cough in February 2002. He had worked in a Eringi (Pleurotus eryngii) mushroom factory since 1996. Chest radiography showed diffuse fine nodular shadows. Chest computed tomography demonstrated centrilobular nodules and increased attenuation in both lungs. The patient suffered from hypoxemia (PaO2 = 65 torr) while breathing room air. The lymphocyte count in the bronchoalveolar lavage fluid was increased, and transbronchial lung biopsy specimens showed lymphocyte alveolitis with epithelioid cell granulomas in the alveolar spaces. After admission, the patient's symptoms improved rapidly without medication. However, on his return to work, fever and hypoxemia appeared again. The lymphocyte stimulating test was positive against extracts of Eringi spores. Precipitins against the extracts of Eringi spores were detected by the double immunodiffusion test. Our diagnosis was hypersensitivity pneumonitis (HP) caused by Eringi spores. In Japan, more than 30 cases of HP induced by mushroom spores have been reported. It is a matter of great urgency to prevent the occurrence of occupational HP in mushroom factories.