Context: The prognostic value of the 6th edition AJCC/UICC TNM classification is currently unclear. Objective: The aim was to evaluate the prognostic value of the 6th edition. Design and Patients: We retrospectively assessed 354 primary differentiated thyroid carcinomas (77 men and 277 women, age: 51.2 yr, follow-up: 107.6 months) between 1964 and 2003. Sixty percent of patients underwent lobectomy, 40% underwent subtotal/total thyroidectomy, and only 2% were given radioiodine. There were 153, 104, 86, and 11 patients in 5th stages I, II, III, and IV, and 175, 76, 14, 68, 10, and 11 patients in 6th stages I, II, III, IVA, IVB, and IVC, respectively. Results: New T1-3 had no significant influence. In Cox proportional hazard analysis, T4a and T4b were significantly related to disease-specific survival (DSS). We separately analyzed 68 patients (45) with extrathyroid extension. These T4 (5th) tumors were reclassified as 6 T3, 52 T4a, and 10 T4b tumors. The 10-year DSS rates were 100%, 69.3%, and 10.0% for T3, T4a, and T4b, respectively. T4b exhibited worse prognoses compared with T4a (P < 0.0001, Hazard ratio 10.1, 95% confidence interval 4.1-25.3). Stages I and II in both editions achieved favorable prognoses. The 10-year DSS rates were 67.0% and 27.3% in 5th stages III and IV, and 100%, 74.5%, 10.0%, and 27.3% in 6th stages III, IVA, IVB, and IVC, respectively. DSS curves differed significantly between all 6th TNM stages (P < 0.0001). Conclusion: The 6th edition more accurately predicts different outcomes according to the revised criteria for the degree of extrathyroid extension.