Low-dose methotrexate is a commonly prescribed medication for the treatment of rheumatoid arthritis and other rheumatic diseases. Its effectiveness in ameliorating the clinical symptoms and findings has been well established, and adverse effects have been relatively infrequent, particularly compared to other immunosuppressive agents. Pneumocystis carinii pneumonia is most often seen in severely immunosuppressed patients related to the acquired immunodeficiency syndrome or treatment of malignancies with potent cytotoxic agents. However, P. carinii pneumonia can occur in patients receiving low-dose methotrexate concurrent with corticosteroids, nonsteroidal anti-inflammatory drugs, renal impairment, or a combination of factors. We report 1 patient and review 10 additional cases that support this relationship.