Objective: The clinical significance of mass screening for thyroid carcinoma remains unclear. This study was carried out to clarify the value of mass screening for thyroid carcinoma. Materials and Methods: From December 1997 through July 1998, a total of 1,401 subjects who were enrolled to receive breast screening or follow-up examination for breast cancer were included in this study. Thyroid glands were examined by 10 MHz ultrasonography by one experienced radiologist. The patients with thyroid nodules were classified into 2 groups according to their potential risk of malignancy by ultrasonographic findings(high-risk : hypoechogenicity, microcalcification, irregular margin, taller than wider shape). High-risk patients were advised to undergo fine-needle aspiration biopsy and thyroidectomy. The characteristics of the thyroid cancers detected by ultrasonographic mass screening were compared by those of clinical thyroid cancer excluding male patients during the same period. Results: Thyroid nodules were detected in 353(25.2%) of the subjects and 259(73.4%) were listed in the low-risk group and 94(26.6%) in high-risk group. Among 94 patients in the high-risk group, 43 underwent thyroidectomy and 37 turned out to have thyroid carcinomas. Thus, the detection rates for carcinoma were 2.6% of all subject, 10.5% of the detected nodules, 36.4% of the high risk women and 86.0% of the operated cases. The tumor size was significantly smaller in the mass-screening group than in the clinical cancer group(p<0.05). However, there was no statistical differences between two groups in the prevalences of neck node involvement and extracapsular invasion and the patients distributions by AMES score, MACIS score and TNM stage. Conclusion: Ultrasonogrpahic mass screening may be useful for the early detection of thyroid carcinoma in women who are scheduled to have breast examination.