Use of methotrexate to treat rheumatoid arthritis is associated with pulmonary adverse effects in 3% to 5% of cases. In addition to immunoallergic lung disease, bronchitis and pneumonia due to pyogenic organisms, opportunistic lower respiratory tract infections have been reported, including, to our knowledge, 18 cases of Pneumocystis carinii pneumonia. We report two new cases of P. carinii pneumonia in methotrexate-treated rheumatoid arthritis patients. One case occurred in a 62-year-old woman with a nine-year history of seropositive rheumatoid arthritis treated for the last seven months with methotrexate, 15 mg per week, and prednisone, 10 mg/d. The other patient was a 58-year-old woman who had been diagnosed with rheumatoid arthritis 18 months earlier and had been receiving 15 mg per week of methotrexate for eight months in combination with 12.5 mg of prednisone per day. Both patients had negative tests for the human immunodeficiency virus. Symptoms consisted of fever, cough and dyspnea, with interstitial infiltrates on chest films, hypoxia, and lymphopenia (700 and 600/mm3, respectively). The diagnosis was confirmed by bronchoalveolar lavage. Both patients recovered under treatment with trimethoprim-sulfamethoxazole. An analysis of the 20 cases of P. carinii pneumonia reported to date in methotrexate-treated rheumatoid arthritis patients demonstrated a number of characteristics: the rheumatoid arthritis was of recent onset in some cases (a few months in one patient); lymphopenia was present in two thirds of cases; one-third of patients were not receiving corticosteroid therapy; the dosage and duration of methotrexate therapy varied widely, from 5 to 30 mg per week and two to 48 months; and four patients died.