AIMS: Estimation of prevalence of autoimmune thyroid disorders in Greek breast cancer patients (prospective study). METHODS: The prevalence of autoimmune thyroiditis was estimated in 310 Greek breast cancer patients, in 100 women with benign breast disease and in 190 women without any breast disease, by submitting them to clinical examination, ultrasound thyroid evaluation, serum thyroid antibody determination and fine needle aspiration (FNA) of the thyroid gland. RESULTS: Autoimmune thyroiditis was found in 136/310 (43.9%) breast cancer women: 95 were diagnosed by positive autoantibodies, 19 had positive FNA findings and 22 had both positive autoantibodies and positive FNA findings. In 117 cases, thyroid autoantibodies were positive (37.7% whereas the control groups had respective rates of 19% and 18.4% autoantibody positivity). CONCLUSIONS: There is evidence of high incidence of autoimmune thyroiditis in Greek breast cancer patients, increasing in relation to cancer stage. Copyright 2001 Harcourt Publishers Limited.