The prevalence of autoimmune thyroid diseases and autoantibodies in patients with chronic fasciolosis (CF) was investigated. Thyroid function of 32 patients with CF and 72 patients without fasciolosis infection was evaluated biochemically. Thyroid hormones, antithyroglobulin antibodies (TgAb) and antithyroid peroxidase antibodies (TPOAb) were measured. In patients with CF, the prevalences of autoantibodies against thyroid peroxidase and thyroglobulin were 21.9% (7/32) and 15.6% (5/32) respectively. However, thyroid function disorder was identified in only 3 (9.4%) cases. In the control group, TPOAb and TgAb were determined in 2 (2.7%) cases, but no one had thyroid function disorder. The prevalences of thyroid autoantibodies (chi 2: 7.948, p < 0.01) and autoimmune thyroid diseases (chi 2: 4.007, p < 0.05) were significantly higher in the CF group. The high prevalence of TPOAb and TgAb in CF suggests that patients with CF should be evaluated for autoimmune thyroid disorders at the time of diagnosis and during the follow-up period.