Methotrexate is a folic acid antagonist that has been used as an antineoplastic and antiinflam-matory drug. Methotrexate pneumonitis is an unpredictable and sometimes life-threatening side effect of methotrexate therapy. Lung lesions associated with this compound include interstitial infiltrates, granulomas, and changes consistent with diffuse alveolar damage associated with perivascular inflammation.