P. carinii was first identified as a human respiratory pathogen in1909 by Carlos Chagas. Over the last twenty years, P.carinii has gonefrom being a rather rare pathogen to be one of the most common causesof pneumonia in immunocompromised hosts, especially AIDS patients. P.carinii pneumonia is the most common cause of death in AIDSpatients. Extrapulmonary infection occur more in AIDS rather thannon-AIDS immunosuppressed patients.Anti-pneumocystis prophylaxis in AIDS patients has asignificant beneficial impact on survival, quality of life, hospitalizationfrequency, and patient’s health care expenditures. TMP-SMX is theagent of choice for prevention of Pneumocystis infection. Pentamidineis an alternative regimen available for patients intolerant to TMP-SMX.