The pulmonary complications of 70 patients with the acquired immunodeficiency syndrome (AIDS) are reviewed. Pneumocystis carinii pneumonia (PCP), present in 67 per cent of the patients, was diagnosed by fiberoptic bronchoscopy with transbronchial biopsies in all of the patients except two adults, who required open lung biopsy, and two children, in whom the infection was detected only at autopsy. Other opportunistic infections, such as cytomegalovirus pneumonitis, mycobacterial infections, invasive candidiasis, toxoplasmosis, cryptococcosis, and histoplasmosis, were more difficult to diagnose by fiberoptic bronchoscopy. In only four cases were these conditions detected during life. Neoplasms and lymphoproliferative processes also presented diagnostic problems, and only one case each of Kaposi's sarcoma and lymphoid interstitial pneumonitis were detected by fiberoptic bronchoscopy. In four other cases these conditions, as well as two pulmonary lymphomas, diffuse large cell immunoblastic type, were detected only at autopsy. Sixty-eight per cent of the patients in this study died, usually with progressive intractable respiratory failure and pulmonary complications that had not been diagnosed during life, including potentially treatable diseases, such as bacterial pneumonias, PCP, nontuberculous mycobacteria, invasive candidiasis, toxoplasmosis, and invasive aspergillosis. The need for earlier detection of pulmonary complications in patients with AIDS is discussed.