Thyroid function was evaluated in 58 patients with breast cancer and compared with data obtained from an age-matched control group of healthy women. Thyroid antibodies at low levels were present in 29 patients (50%): 21 patients (36%) had thyroid stimulating antibodies at low activity. Ten patients (17%) had thyroglobulin antibodies compared with 3 in the control group (P less than 0.05). Nine of 10 patients with thyroglobulin antibodies had microsomal antibodies as well. Two patients had only microsomal antibodies. Additionally 10 autoimmune antibodies were determined. These were, however, either absent or present in low concentrations in serum. Independent of the stage of disease, no differences were demonstrated between patients and controls in serum levels of triiodothyronine, thyroxine, free T3 index, free T4 index, thyroid stimulating hormone, and thyroglobulin. In conclusion, we found an increased frequency of thyroid autoantibodies in euthyroid patients with breast cancer compared with healthy controls suggesting a possible relation between this disease and autoimmune thyroid disease.