We report a 49 years old woman with systemic lupus erythematosus and a WHO type IV nephropathy, treated with prednisone 1 mg/kg/day po and cyclophosphamide 1 g/month iv. After two months in this treatment schedule, she presented with an acute pneumonia; broncoalveolar lavage and lung biopsy disclosed the presence of Pneumocystis carinii. She was treated with trimethoprim-sulfamethoxazole 960 mg tid with a favorable response. Opportunistic infections are frequent in lupus erythematosus and Pneumocystis carinii pneumonia has been recently reported in this disease. The changes in immune response and the adverse effects of drugs used in its treatment may explain the increased susceptibility of these patients to infections by Pneumocystis carinii.