Despite a declining prevalence secondary to improved prophylaxis, Pneumocystis carinii remains an important pulmonary pathogen in the immunocompromised host. Because the radiologist is often the first to suggest the diagnosis of PCP, an awareness of the entire spectrum of imaging features associated with this organism is important. The classic presentation of PCP is a bilateral interstitial pattern, which may be characterized as finely granular, reticular, or ground-glass opacities. When chest radiographic findings are normal or equivocal, high-resolution CT may be helpful, because it is more sensitive than chest radiographs for detecting PCP. The classic CT finding is extensive ground glass attenuation. Increasingly recognized characteristic patterns of PCP in AIDS patients include cystic lung disease, spontaneous pneumothorax, and an upper lobe distribution of parenchymal opacities. Although the radiographic findings in PCP are similar for AIDS and non-AIDS immunosuppressed patients, cystic lung disease has not been described in the latter patient population.