PURPOSE: To determine the usefulness of CT for diagnosing metastases to primary and secondary echelon lymph nodes (LNs) and to investigate various CT findings of metastatic LNs in thyroid carcinomas. MATERIALS AND METHODS: We retrospectively reviewed the CT and histologic findings in 59 patients with thyroid carcinomas who had undergone thyroidectomy and neck dissection. Primary echelon LNs (Level VI) were removed by central neck dissection in all patients, and in 21, a total of 136 levels of secondary echelon LNs (Level II-V) were excised away by lateral neck dissection. CT criteria of metastatic LNs included large size, significant homogeneous enhancement, calcification, and cystic change. We evaluated the ability of CT to detect primary and secondary echelon LN metastasis and tried to determine which CT features were useful for the diagnosis of LN metastasis. RESULTS: Histologically, LN metastasis was found in 31 (53%) of 59 patients, including 30 with metastasis to primary echelon LNs. Of the 136 levels of secondary echelon LNs resected in 21 patients, 44 were found at histology to harbor metastatic foci. The sensitivities, specificities, positive and negative predictive values, and accuracies of CT in the diagnosis of metastasis to primary and secondary echelon LNs, respectively, were 27% and 93%, 100% and 93%, 100% and 87%, 57% and 97%, and 63% and 93%. While all secondary echelon LNs with at least one of the following CT criteria-large size (n=19), cystic or necrotic change (n=14), or calcifications (n=8)-were histologically proven to be metastatic, six (24%) of 25 such LNs with a sole sign of significant enhancement at CT were found to be due to reactive lymphadenopathy. CONCLUSION: Although CT was unable to detect metastasis to primary echelon LNs, it was useful in the detection of secondary echelon LN involvement. Large size, cystic change, and calcification are considered highly reliable signs of metastatic LNs.