PURPOSE: A relationship between thyroid disease and primary breast cancer remains controversial. Several studies have demonstrated there is an increased risk of breast cancer in patients with thyroid dysfunction and thyroid cancer. The purpose of this study was to evaluate the incidence, relationship and clinical characteristics of thyroid nodules and cancer in patients with breast cancer. METHODS: Two hundred sixty one women ith primary breast cancer had an ultrasonogram of the thyroid prospectively performed at the Department of General Surgery of the Soonchunhyang University Hospital from April 1995 to December 1999. The thyroid nodules were identified by high-resolution ultrasonography before all patients had undergone breast cancer surgery. RESULTS: Among the 261 patients with breast cancer, thyroid nodules were found in 57 (21.8%). The most prevalent age group of the 57 thyroid nodules was the 5th decade. Twenty-one out of 57 patients with thyroid nodules had undergone thyroid surgery and 14 benign nodules and 7 thyroid cancers (5 papillary carcinoma & 2 follicular carcinoma) were discovered. The incidence of thyroid cancer in the patients with breast cancer was 2.7%. CONCLUSION: Although the factors associated with thyroid nodules and cancer in the patients with breast cancer were not demonstrated, these results were suggested that there may be a relationship between them. Therefore, ultrasonography of the thyroid in patients with breast cancer priorto breast cancer surgery operations is recommended.