A 59-year-old man presented with fever and exertional dyspnea in August 2008. He was diagnosed as having acute hypersensitivity pneumonitis (HP) on the basis of thoracic computed tomography, bronchoalveolar lavage (BAL), and transbronchial lung biopsy. Serum anti-Trichosporon antibody was negative then. He was suspected to have farmer’s lung because his neighbor was a farmer. Steroid therapy was started, and his symptoms had improved. Because fever and dyspnea recurred during tapering of steroid, he was admitted to our hospital. After environmental investigation, several diseases including isocyanate-induced HP, farmer’s lung, bird-related HP, and summer-type HP had to be differentiated. We reexamined specific antibodies both in serum and BAL fluid. He was diagnosed as having summer-type HP because of positive anti-Trichosporon antibodies and a positive home-returning provocation test. Environmental investigation and a strict provocation test were helpful for the diagnosis of HP.