Pulmonary fibrosis is an extraarticular manifestation of rheumatoid arthritis (RA), which appears between 20 to 40% of cases. MTX is an ordinary treatment used for patients with RA. It has been suggested in some studies that a link between the use of MTX and the appearance of pulmonary fibrosis exists. Furthermore, the chronic use of MTX has been associated with the development of hepatic cirrhosis. Adenosine, a key molecule in the anti-inflammatory action of MTX seems to have a pro-fibrotic effect in some experimental models, thereby suggesting a mechanism through which MTX could lead to a fibrotic process. However, in spite of these experimental models, clinical studies have not permitted to confirm the pathogenic role of MTX in pulmonary fibrosis.