In 15 patients with rheumatoid arthritis (RA) (n = 13) or psoriatic arthritis (PsA) (n = 2) p-homocystcine and erythrocyte folate (erc-FA) were measured before start of methotrexate (MTX) treatment, after 4 weeks of MTX treatment (median 10 mg per week), and after further 4 weeks of treatment with MTX (median 12.5 mg per week) supplemented with folic acid (FA) (15 mg per week). Mean p-homocysteine were 12.3 +/- 3.4 micromol/l, 14.6+/-5.8 micromol/l (p<0.05) and 10.3+/-3.0 micromol/l (p<0.01) respectively. P-homocysteine concentrations were negative correlated to erc-FA after 4 weeks (rho -0.58; p<0.05). It is concluded that treatment with MTX induces a significant rise in p-homocysteine that is neutralised by FA supplementation. Supplementation with FA from the start of MTX treatment is recommended considering the increased risk of cardiovascular disease that is associated with elevated concentrations of p-homocysteine.