In this study serum homocystein (Hcy) level was measured and its relationship with disease activity criteria and treatment protocols was investigated in ankylosing spondylitis (AS) patients. Ninety-two AS patients and 58 healthy individuals were recruited. Erythrocyte sedimentation rate and serum C-reactive protein were determined. Bath AS disease activity index and Bath AS functional index were calculated. Serum Hcy levels >15 micromol/l were considered as hyperhomocysteinemia. The mean serum homocysteine levels were 14.40 and 12.60 micromol/l in patients with AS and the control group, respectively, and the difference between two groups was significant. While there was no significant difference between the sulfasalazine (SSZ) group with 14.25 micromol/l mean Hcy level and the methotrexate (MTX)/SSZ group with 16.05 micromol/l, there was a statistically significant difference between the Hcy levels of these two groups and Hcy level of 12.15 micromol/l of the non-steroidal anti-inflammatory drugs group, and 12.60 micromol/l Hcy level of the control group. Mean serum Hcy level was 13.65 micromol/l in patients with active AS and 14.60 micromol/l in patients with inactive AS, and there was no significant difference between the groups. In our study serum Hcy level was found to be significantly higher in patients with AS than in healthy control subjects. Especially for the AS patients receiving MTX and SSZ treatment without folic acid supplementation, addition of folic acid to their therapy may decrease the risk of cardiovascular disease which in turn decreases the mortality in these patients, but further prospective studies are needed for supporting these results.