From 1982 to 1987, 22 patients with proven Pneumocystis carinii pneumonia were diagnosed at Wellington Hospital. Patients comprised 15 males and 7 females aged 15-76 years and included seven with AIDS, eight with haematological malignancy and seven with renal disease. Two distinct clinical prodromes occurred. In renal patients a classic fulminating pneumonitis developed over 24 to 72 hours. In patients with AIDS a more indolent illness occurred lasting 3 or more weeks and was characterised by fever, dry cough and breathlessness. Haematology patients showed no specific duration of prodrome. At the time of diagnosis all had an abnormal chest radiograph and the arterial PO2 was reduced in all but one case. An invasive diagnostic procedure was performed in all except one case where the diagnosis was made at post mortem. Two patients required a second procedure to establish the diagnosis. Procedures performed included bronchoalveolar lavage [14], open lung biopsy [7] and transbronchial lung biopsy [2]. All patients were treated with high dose cotrimoxazole and 18 survived to leave hospital. A review of the approach to the diagnosis and treatment of Pneumocystis carinii pneumonia is presented.