Pneumocystis carinii is a common cause of pneumonia in individuals who are immunosuppressed by HIV infection. Use of molecular biological techniques show that P. carinii is a fungus and that infection in man is not a zoonosis. Invasive tests such as sputum induction or bronchoscopy are used to make the diagnosis of P. carinii pneumonia. Life long primary prophylaxis is given to HIV positive individuals with CD4+ lymphocyte counts < 0.20 x 10(9)/L or a CD4: total lymphocyte ratio of < 1.5, constitutional symptoms, or with other AIDS defining diseases. Secondary prophylaxis is given after a first episode to prevent a recurrence. First choice for primary and secondary prophylaxis is oral co-trimoxazole 960 mg od or three times a week. In patients who are intolerant to co-trimoxazole, nebulised pentamidine or dapsone (with or without pyrimethamine) are second and third choices. In a patient with acute PCP disease, severity should be assessed using clinical, radiographic and blood gas criteria as those with moderate or severe disease will benefit from adjuvant glucocorticoids. Co-trimoxazole (120 mg/kg/day in divided doses for 21 days) is first choice therapy for PCP of all degrees of severity. In patients who fail to respond to co-trimoxazole or who are intolerant to it, second line treatment is iv pentamidine in those with severe disease and oral dapsone with trimethoprim, oral clindamycin with primaquine or iv pentamidine in those with mild or moderately severe disease.