Two patients with haematologic malignancies developed Pneumocystis carinii pneumonia while under outpatient treatment, one on busulphan for chronic myelogen leukemia, and the other on prednisone plus chlorambucil for non-Hodgkin's lymphoma. The first patient was moderately ill and required hospitalization for 12 days while the second patient was critically ill and needed assisted ventilation for two weeks. Eventually they both recovered and returned to work. Tests for serum antibodies to the human immunodeficiency virus (HIV) were negative in both patients. We review the problem of P. carinii pneumonia in patients receiving immunosuppressive drugs.