A 73-year-old man with mild inactive interstitial pneumonitis and non-small cell bronchogenic carcinoma developed a cell-mediated hypersensitivity pneumonitis within 15 days following a second exposure to paclitaxel. Histological features of transbronchial lung biopsy were consistent with interstitial pneumonitis. Bronchoalveolar lavage showed increased lymphocytes and eosinophils, and decreased helper/suppressor T lymphocyte ratio. Leukocyte migration inhibitory test was positive for paclitaxel. These results support the possibility that paclitaxel-induced pneumonitis is due to delayed type hypersensitivity; they also support the hypothesis of a central role of T lymphocytes in drug-induced pneumonitis.