No consensus for papillary carcinoma of the thyroid exists on the preoperative diagnosis of lateral cervical lymph node metastasis, indications, or range of neck dissection, so we studied the usefulness and limits of ultrasonography and sufficient dissection by comparing preoperative ultrasonographic and postoperative histopathological diagnosis. Subjects were 45 patients (51 affected sides) with lateral cervical lymph node metastasis of papillary carcinoma of the thyroid who underwent modified neck dissection between July 1997 and July 2003. Preoperative ultrasonographic and postoperative histopathological diagnosis were compared. Specimens excised by neck dissection contained 1,325 lymph nodes. Of these, 198 (15%) detected by preoperative ultrasonography were selected for investigation of diagnostic criteria for metastasis-positive lymph nodes. The best criterion for the diagnosis of metastasis-positive lymph node was 0.5 or greater [minor axis/major axis] with 6 mm or greater minor axis at levels III, IV, or V (7 mm or greater at level II), and sensitivity, specificity, and accuracy were 78%, 100%, and 84% respectively. The lateral cervical lymph node metastasis rate obtained by this diagnostic criterion was 41%. Regional histopathological metastasis positivity was investigated in the lateral cervical region, and high positivity rates were obtained: 57% at level II, 71% at level III, and 84% at level IV. Considering these findings and the preoperative ultrasonographic diagnosis rate of 41%, sufficient dissection at levels II-IV may be necessary for patients in whom lateral cervical metastasis is observed before surgery. The metastasis rate was 10% at level V, but dissection should always be done in lateral cervical metastasis-positive patients because: 1) No trend was observed in age, gender, the number of metastatic lymph nodes, or regional metastasis rate; 2) no anatomical boundary is present between levels II, III, IV and level V; 3) no functional disorder due to preservation of the accessory nerve occurred; 4) the prognosis of patients with advancement to the accessory nerve was poor; and 5) improvement of the prognosis of papillary carcinoma of the thyroid by modified radical neck dissection has been reported.