Computed tomographic (CT) scans of the chest in 39 patients with Pneumocystis carinii pneumonia (PCP) were reviewed to determine the spectrum and frequency of CT manifestations of PCP. Parenchymal disease was categorized as either bilateral or unilateral, symmetric or asymmetric, and diffuse or patchy. Infiltrates were classified as interstitial, airspace, or mixed. On this basis, three CT patterns of involvement were identified: a ground-glass patchwork pattern in 22 of 39 (56%), and an interstitial pattern in seven of 39 patients (18%). Atypical CT features of PCP included nodules and nodular components in seven of 39 (18%) and cavities in three of 39 patients (8%). Associated CT findings included cystic spaces and bullae in 15 of 39 (38%), pneumothorax in five of 39 (13%), adenopathy in seven of 39 (18%), and pleural effusions in seven of 39 patients (18%). It is concluded that although PCP may exhibit a variety of CT appearances, certain patterns are more common than others. In the appropriate clinical setting, these findings are highly suggestive of PCP. The identification of cavities or nodular components in addition to infiltrates should raise the suspicion of a second disease process or mixed infection affecting the lungs.