In 1993, a hundred and fifty AIDS patients were submitted to high-resolution CT (HRCT). In 102 patients, bronchoalveolar lavage and/or transbronchial biopsy findings suggested the diagnosis of Pneumocystis carinii pneumonia--a pure Pneumocystis carinii infection in 75 patients and associated with other pathogenic agents in 27. We report the most common HRCT patterns, such as ground-glass opacities, cysts, interstitial changes and nodules. Ground-glass opacities were demonstrated in 57.8% of cases, cysts in 44.1%, interstitial involvement in 52.9% and nodules in 28.4%. HRCT permitted lung disease to be demonstrated in 55% of our patients, suffering from impaired breathing, with negative chest films. Respiratory function tests and gallium scintigraphy show their low specificity in the diagnosis of Pneumocystis carinii infection because, although depicting diffuse interstitial involvement, they fail to detect the pathogenic agent. As for hemogasanalysis, in the presence of hypoxia, this technique can suggest the diagnosis of Pneumocystis carinii infection, while the pathogenic agent can be isolated with bronchoalveolar lavage, which demonstrates the simultaneous decrease in CD4 and increase in CD8 lymphocytes, respectively. To conclude, HRCT does detect the basic changes occurring in Pneumocystis carinii pneumonia, thus contributing to the diagnosis of this condition.