A 47-year-old automobile painter developed dry cough, chills and dyspnea after four months of painting work. The spray contained isocyanates (HDI). A chest radiograph showed bilateral ground-glass shadows. The serum KL-6 level was very high: 11,100 U/ml. Marked lymphocytosis and a striking decrease in the CD4/CD8 ratio were observed in the bronchoalveolar lavage fluid. Transbronchial lung biopsy specimens showed alveolitis and bronchiolitis. Cessation of exposure to isocyanates improved the symptoms and laboratory data. We suspected that the patient was suffering from hypersensitivity pneumonitis induced by isocyanates. Specific IgG antibodies for TDI, MDI, and HDI were not demonstrated by enzyme-linked immunosorbent assay. Because abnormal shadows in the chest radiograph did not improve quickly after admission, we administered glucocorticoids which improved the symptoms, the chest radiograph findings, and the serum KL-6 level.