Three of 95 patients with rheumatoid arthritis who were being treated with low-dose (5 to 15 mg/wk) methotrexate sodium developed the clinical, radiographic, and pathologic features of methotrexate-associated pulmonary injury. Marked hypoxemia emphasized the severity of illness in our patients; lowest oxygen pressure values for each patient were 35 mm Hg, 42 mm Hg, and 45 mm Hg. The management of our patients with a pulmonary toxic reaction to methotrexate included discontinuing the drug treatment, antibiotic therapy until an infectious cause was excluded, and high-dose methylprednisolone. Two patients recovered and one died. Contrary to an earlier report that suggested that pneumonitis occurred only with methotrexate sodium doses exceeding 15 mg/wk, our three cases demonstrate that a severe pulmonary toxic reaction may also complicate low-dose weekly methotrexate therapy of rheumatoid arthritis.