Methotrexate (MTX) is a folic acid analogue with antiproliferative and antiinflammatory effects. In the past several years, MTX has become the most commonly used agent in patients with severe, destructive psoriatic arthritis (PsA), with positive clinical results. Liver changes and serum enzyme level increases do not seem to be a major problem in PSA patients treated with MTX. In addition, PSA patients treated with low-dose MTX were not associated with pulmonary fibrosis as evaluated by means of sensitive imaging techniques and pulmonary function tests. The concomitant use of folic acid reduces both the frequency of serum liver enzyme level increases but also the efficacy of MTX by competing with the folate receptors.