To determine plasma homocysteine levels in ankylosing spondylitis (AS) and their correlation with disease activity measurements. To examine the effects of different treatment modalities on homocysteine levels. One hundred eight patients diagnosed with AS according to New York criteria and 65 healthy individuals matched in terms of age and gender were enrolled in the study. Patients were given detailed physical examinations. The Bath AS Disease Activity Index (BASDAI) was used for disease activity, the Bath AS Metrology Index (BASMI) for spinal mobility, the Bath AS Functional Index (BASFI) to determine functional status and the Bath AS Radiological Index (BASRI) for radiological analysis. Sedimentation rate (ESR) and C reactive protein (CRP) levels, acute phase reactants, were measured. Plasma homocysteine levels, serum interleukin (IL) -6 and serum tumor necrosis factor- alpha (TNF- alpha) levels were investigated using the enzyme-linked immunosorbent assay (ELISA) method. Plasma homocysteine levels in AS patients were statistically significantly lower compared with those in the healthy control group (P = 0.0001). There was no significant difference among sub-groups established on the basis of medical treatments and disease activity (BASDAI a parts per thousand currency sign4 and > 4). No statistically significant correlation was determined between homocysteine level and disease activity parameters, radiological score and functional indices. A significant negative correlation was, however, established between age and homocysteine level in the AS group (P < 0.05, r = -0.426). Plasma homocysteine was lower in AS patients compared with the control group. This is not correlated with disease activation and medical treatment employed.