PURPOSE: To determine the prevalence of thyroid nodules discovered incidentally at ultrasonography and to reassess their significance. MATERIALS AND METHODS: During a six-month period and using a high-frequency transducer, 1033 subjects with suspected breast disease underwent a prospective study of the thyroid. We determined the prevalence of thyroid nodules according to age, and the malignancy rate. RESULTS: Focal thyroid nodules were detected in 291 women (28.2%), their prevalence increasing with age (p<0.01). The nodules, totalling 337, were single in 260 cases (89.3%) and multiple in 31 (10.7%); 271 were less than 1 cm in diameter, 53 were 1-2 cm, and 13 were more than 2 cm [mean, 7.1(range, 2-34) mm]; 126 (37.4%) were pure cysts, 62 (18.4%) were mixed, and 149 (44.2%) were solid. Of the 149 solid nodules, 139 were hypoechoic. Eighty women (with 106 nodules) underwent fine-needle aspiration bipsy, and 35 (with 45 nodules) underwent surgery. It was discovered, finally, that 29 women (2.8%) had 35 malignancies, of which 33 were papillary carcinomas and two were carcinomas which had metastasized from a breast malignancy. Eleven women (37.9%) had extrathyroidal invasion and 6 (20.7%) had cervical lymph node metastasis despite incidentaloma. CONCLUSION: The prevalence of incidentally found thyroid nodules was 28.2%. Since the malignancy rate of these was relatively high, at 10.4%, the clinical significance of this finding should be reassessed.