A 46-year-old man who had worked in a paint processing plast for over 29 years was admitted to our hospital with complaints of nocturnal dyspnea and dry cough. A chest X-ray film showed diffuse granular shadows in bilateral lungs. Pulmonary function tests revealed reduction of diffusing capacity and restrictive impairments. Hypersensitivity pneumonitis (HP) due to isocyanates was speculated from his occupational history and clinical course. Positive skin tests against TDI-HSA and MDI-HSA, precipitating antibody against TDI-HSA, and negative lymphocyte stimulating tests of peripheral blood and bronchoalveolar lavage fluid were also noticed. Environmental provocation test was positive. Histological findings of transbronchial lung biopsy specimens showed diffuse alveolitis and Masson body, but no granulomas. According to these results, the patient was diagnosed as HP due to TDI. Type III allergy of Gell-Coombs seems to participate in this case. The granulomatous lesion is seen less frequently in isocyanate-related HP than in HP induced by organic dusts, which suggests the difference in immunological and histological reactions between both types of HP.