Over the last decade the number of subjects with acquired immunodeficiencies has markedly increased; this phenomenon depends on both the large number of patients receiving organ transplants or antiblastic therapy and the spread of infections caused by the HIV virus. In 70-90% of these patients primary diseases include different pulmonary infections, relative to the type and degree of immune compromission. Pathogenic or, in most cases, opportunistic germs are responsible for severe pneumonia whose mortality rate can top 50%. Since prognosis depends on the promptness of treatment, the diagnosis of nature must be made quickly by integrating clinical and diagnostic findings with laboratory and instrumental results. Conventional chest radiology plays a major role as the first step in a diagnostic iter which can now include rather sensitive techniques--e.g., equalized chest films. CT and nuclear medicine often represent the necessary diagnostic complements but, in some cases, etiology can be diagnosed only with such invasive procedures as lung biopsy. The authors reviewed the current data on the diagnostic imaging findings of pulmonary infections caused by common germs, by Pneumocystis carinii mycobacteria, mycetes and viruses in immunocompromised patients, integrating their personal experience with literature data.