The risk of breast cancer associated with thyroid supplementation for iatrogenic hypothyroidism was investigated in women treated for hyperthyroidism at the Mayo Clinic, Rochester, Minn. One or more years after the diagnosis of hypothyroidism, there were 47 cases of breast cancer observed among 1,665 women compared with 52.9 cases expected, a ratio of observed to expected cases (standardized incidence ratio [SIR]) of 0.9. There was also no breast cancer excess among women who remained euthyroid (SIR=1.0). Breast cancer risks were not influenced by various factors associated with the management of hyperthyroidism or subsequent hypothyroidism. No increased risk was observed in hypothyroid women categorized by breast cancer risk factors except for women who reported a previous breast biopsy, an excess also observed among euthyroid patients. The results of this study indicate that use of thyroid supplements does not increase the risk of breast cancer developing.