The increased sensitivity of many imaging devices has increased the identification of asymptomatic nodules in the thyroid gland. In this study we investigated the actual incidence of nonpalpable thyroid nodules and occult carcinoma in women. Between January and June 2003, among the women who were scheduled to undergo breast ultrasonography, 697 without palpable thyroid nodules were screened for thyroid nodules. They were classified into four categories according to ultrasonographic findings: negative, benign, indeterminate and malignant. Ultrasound-guided fine-needle aspiration biopsy (FNAB) was carried out for all malignant lesions and for some of the benign and indeterminate ones. The nodule detection and malignancy rates were determined and the effectiveness of ultrasonography as a diagnostic tool was also investigated. Out of the 697 subjects, 246 (35.3%) were found to have thyroid nodules. The malignancy detection rate based on the FNAB results, including both suspicious and malignant groups, was 3.6% (25/697) for all subjects. In addition, 3.0% (21/697) of all the women were confirmed to have thyroid cancer by surgery. The sensitivity of sonographic classification was increased from 80% to 100% when the indeterminate class was added to the malignant one, although this decreased the specificity from 91.7% to 33.3%. In conclusion, high-resolution ultrasonography detected a high percentage of malignant nodules. Ultrasonography can augment its value by guiding FNAB, in addition to providing diagnostic images.