Patients with systemic lupus erythematosus (SLE) have increased susceptibility to infection by Pneumocystis carinii, but this condition has rarely been reported in Taiwan. Here, we describe two cases of patients with SLE who developed Pneumocystis carinii pneumonia (PCP). The first patient was a 39-year-old woman presenting with fever and dyspnea that had lasted 2 weeks. Chest roentgenography disclosed bilateral interstitial and alveolar infiltrates. The second patient was a 22-year-old woman presenting with a 4-day history of malaise, cough, dyspnea, and fever. She had concomitant Mycobacterium tuberculosis infection. Both patients had been treated with varying doses of corticosteroids and/or cytotoxic drugs within 4 months before presentation. Diagnosis was established based on the findings of bronchoalveolar lavage (BAL) and transbronchial lung biopsy (TBLB). Both patients received trimethoprim-sulfamethoxazole (20 mg.kg-1.d-1 trimethoprim), but finally died of nosocomial septicemia (Acinetobacter baumanni and Pseudomonas aeruginosa bacteremia in one, P. aeruginosa bacteremia in the other). These two cases demonstrate that PCP should be included in the differential diagnosis of patients with SLE presenting with pneumonic processes. In addition, a second opportunistic pathogen should be suspected. Bronchoscopic examination should be performed if the diagnosis is not clear and should include TBLB and BAL.