A 79-year-old man was given a cumulative dose of 16.5 g of nilutamide for treatment of prostate cancer. He then presented with a respiratory illness having clinical, radiologic and functional characteristics of interstitial pneumonitis. No other cause of pneumonitis was found. Bronchoalveolar lavage showed a lymphocytic alveolitis with an inverted lymphocyte subset ratio. After an 11-week period of drug withdrawal, clinical, radiologic and functional improvement was observed along with a normal alveolar lymphocytosis. Nilutamide therapy was then resumed for five weeks and induced the recurrence of clinical, functional and alveolar abnormalities. Nilutamide treatment was finally stopped and two months later, clinical and functional abnormalities resolved. This observation seems to exemplify the possible diagnostic value of coupling provocation test with BAL cell data in hypersensitivity pneumonitis induced by drugs. In addition, these data support the role of a cell-mediated immunologic mechanism in the pathogenesis of nilutamide-induced pneumonitis.