Although autoimmune thyroid disease is well known to be associated with primary Sjogren's syndrome (SjS) and with various autoimmune diseases, it is less clear whether a similar association also exists for ankylosing spondylitis (AS). Therefore, we investigated the frequency of autoimmune thyroid disease in patients with AS. In this cross sectional study, 80 patients with AS fulfilling the 1984 Modified New York Criteria and 80 healthy subjects, age and sex-matched with AS patients, were included. As the positive control group, 62 female patients with primary SjS were also studied. All cases underwent thyroid ultrasonography (USG) by a single endocrinologist. Thyroid function tests and thyroid autoantibodies were measured. The diagnosis of Hashimoto's thyroiditis (HT) was made if the patient had thyroid autoantibody positivity plus at least one of the following criteria: diffuse goiter with physical examination, abnormality in thyroid function tests, and parenchymal heterogeneity with USG. The chi-squared test and Fisher's exact test were used to compare cases and controls. The p values < 0.05 were considered statistically significant. The frequencies of parenchymal heterogeneity with USG (30 vs 11.3 %, p = 0.045), thyroid autoantibody positivity (13.8 vs 2.5 %, p = 0.017), and concomitant diagnosis of HT (10 vs 1.3 %, p = 0.034) were significantly higher in AS group compared to healthy controls. Among AS patients having HT, subclinical hypothyroidism was detected only in a single patient. Frequency of autoimmune thyroid disease was significantly higher in AS group, compared to healthy controls. Prospective studies are needed to see the clinical relevance of these findings and outcome in the long term.