Pneumocystis carinii pneumonia (PCP) is one of the leading complications among HIV-infected patients. Recent advances in PCP prophylaxis, diagnosis and treatment have caused a decrease in PCP-related morbidity and mortality. Despite these advances, PCP continues to be frequent in patients not known to be HIV-infected and in those patients with poor adherence to prophylactic regimens or severe immunosuppression. In typical cases diagnosis may be suspected by the patient's clinical presentation. Clinicians are frequently faced with the differential diagnosis between PCP, bacterial pneumonia, pulmonary tuberculosis, and other specific respiratory disorders HIV-associated. Definitive diagnosis of PCP requires demonstration of Pneumocystis carinii (PC) in respiratory secretions or lung tissue. Conventional techniques, immunofluorescence using monoclonal antibodies and molecular techniques are highly specific, but sensitivity varies depending on the PC load present in the sample. Best diagnostic yield is obtained analyzing samples obtained by bronchoalveolar lavage. PC diagnosis using highly sensitive PCR in sputum-induced samples might allow noninvasive diagnosis in most HIV-infected patients suffering from PCP but PCR techniques remain to be standardized. Like in PCP prophylaxis, trimethoprim-sulphametoxazole (TS) is the drug of choice for PCP treatment. In severe case, TS is given intravenously. If patient is intolerant to TS, i.v. pentamidine or i.v. trimetrexate with folinic acid can be used. TS has a dose-dependent toxicity. In cases of hypersensitivity to TS, drug-desensitization should be tried. In severe documented PCP adjunctive corticosteroid therapy is effective and safe. In mild to moderate PCP, TS can be given orally. Best alternatives to TS in this situation are dapsone-pyrimethamine or clindamycin-primaquine (CP). Other effective options are oral atovaquone, aerosolize pentamidine and i.v. pentamidine.