OBJECTIVE To establish the prevalence of antiphospholipid antibodies (aPL) in a group of patients with ankylosing spondylitis (AS). The relation of the antibodies with different clinical and analytical features was studied. METHODS Eighty-four patients with AS (71 men) and 40 age and sex matched controls were studied. aPL determinations included: anticardiolipin antibodies (aCL) of the IgM, IgG, and IgA classes, the presence of lupus anticoagulant (LAC), and a false positive serologic test for syphilis. Comparisons between variables were done by Student t test, Mann-Whitney test and Chi squared test. Correlations between aPL and clinical variables were performed by Pearson coefficients. RESULTS Twenty-four patients with AS (29%) has positive IgG aCL, compared with only 2 cases in the control group (5%) (p < 0.002). There were no differences in other aPL determinations between patients and controls. There was no correlation between the presence of aCL (IgG, IgM, or IgA) and LAC and the different aspects of the disease. Two patients fulfilled the criteria for antiphospholipid syndrome. CONCLUSIONS Our results indicate the presence of IgG aCL in patients with AS higher than in the normal population but their relation with thrombosis and other systemic manifestations seems weak.