PURPOSE: To analyze the ultrasonographic (US) findings of thyroid nodules that yielded false negative results after an initial ultrasound-guided fine needle aspiration biopsy (FNAB). MATERIALS AND METHODS: Between August 2003 and February 2006, 389 patients with 405 thyroid nodules received a repeat US-guided FNAB. We retrospectively reviewed the US findings, cytology results and postsurgical pathological results. The cytology diagnoses were classified as benign, a suspicious malignancy, a follicular neoplasm, a papillary carcinoma, and a non-diagnostic result. The US findings of the thyroid nodules were analyzed with regard to size, internal content, shape, margin, echogenecity, and calcification pattern. RESULTS: Of the 405 thyroid nodules, 17 nodules were false negative. The major US findings of these nodules were a solid internal component in 16 nodules, hypoechogenicity or marked hypoechogenicity in 14 nodules, microcalcifications in 12 nodules, an ovoid to round shape in 9 nodules and a well-defined smooth margin in 9 nodules. CONCLUSION: An repeat US-guided FNAB should be performed if the thyroid nodules have one of the malignant US features such as hypoechogenecity or marked hypoechogenecity, a microcalcification, a taller than wide shape or a well-defined spiculate margin although the cytology results indicated a benign lesion. In addition, thyroid nodules with findings of a well-defined smooth margin, ovoid to round shape, and solid internal component might also be subject to a repeat US-guided FNAB to exclude a malignancy.