In the U.S.A., Pnennocystis carinis is one of the most frequently encountered pulmonary pathogens in immunocompromised patients, especially those with AIDS, The rapid identification and Treatment ofP. cariniiinfection is important in the management of these patients. CurrentlyP. carinacannot cultivated in cell-free media, and is usually identified in human infections by microscopic examination of histologic or cytologic specimens. The classic appearance ofP.cariniipneumonia, particularly in human immunodeficiency virus 1 (HIV-1) infected patients, is widely known. However, atypical clinical, radiological and pathological presentations may pose problems in diagnosis One important diagnostic pitfall is thatP.cariniimay cause granulomatous pulmonary reactions which resemble those caused by other pulmonary pathogens, such as mycobacteria and fungi, Awareness of this unusual morphologic variant ofP.cariniiinfection is important in the proper management of such patients.