More than 50% of patients with acquired immunodeficiency syndrome (AIDS) develop pulmonary disease during the course of their illness. The authors reviewed 96 computed tomographic (CT) scans of patients with AIDS in an attempt to describe disease entities by the patterns seen on the scans. Such patterns included isolated ground-glass and interstitial infiltrates, which are suggestive of Pneumocystis carinii pneumonia (PCP). If pleural effusions or parenchymal nodules are also present, AIDS-related lymphoma (ARL) or Kaposi sarcoma (KS) is more likely. Although diffuse alveolar infiltrates are most commonly present in PCP, a segmental alveolar infiltrate is suggestive of a bacterial pneumonia, especially when associated with cavitation or ipsilateral pleural effusion. Well-defined nodules are typical for ARL, whereas ill-defined nodules are more commonly suggestive of KS. Accompanying adenopathy or effusion with nodules further suggests ARL. Different combinations of parenchymal, nodular, and pleural abnormalities may be suggestive for additional diagnoses, including Mycobacterium tuberculosis, M avium-intracellulare, and Cryptococcus neoformans infections and human immunodeficiency virus adenopathy. The authors believe that a specific pattern of involvement can help suggest a likely diagnosis in many instances.