A 35-year-old homosexual man who had a remote history of cocaine abuse presented to the hospital with fever, chills, drenching night sweats, and progressive dyspnea of 3 months' duration. His condition had been diagnosed as AIDS 1 1/2 years before presentation. Multiple blood cultures and serological tests failed to yield an infective etiology. Bronchoscopy with transbronchial biopsy, both performed twice, also failed to reveal an etiology. Empirical treatment for infection with the Mycobacterium avium complex yielded no response; empirical treatment, based on abnormalities revealed by gallium scanning, for Pneumocystis carinii pneumonia led to some clinical improvement. Because of rapid respiratory deterioration at the end of this treatment course, a thoracoscopic lung biopsy was performed; this procedure demonstrated classic bronchiolitis obliterans organizing pneumonia. Corticosteroid therapy resulted in a rapid salutary response. It is important to aggressively pursue a definitive diagnosis for selected patients with a nonidentifiable infectious cause so that patients receive the correct treatment.