We describe 6 cases of patients with systemic lupus erythematosus (SLE) who developed Pneumocystis carinii pneumonia. All were treated with high dose corticosteroids, and all developed the infection within 4 months of beginning new or revised cytotoxic therapy. All patients tested (5 of 6) were negative for human immunodeficiency virus (HIV). Those patients who developed Pneumocystis carinii pneumonia had more severe lymphocytopenia (median 595 vs 833/mm3) and received higher doses of corticosteroids (median prednisone dose = 43 vs 20 mg/day) than other patients with active SLE. A threshold lymphocyte count of 350/mm3 identified 4 of 6 cases but only 1 of 20 controls. Patients with SLE treated with high dose corticosteroids and cytotoxic drugs and with severe lymphocytopenia may be at increased risk for this opportunistic infection.