Background The AJCC/UICC TNM staging system (TNM) is a widely accepted system for differentiated thyroid carcinoma (DTC). The objective of the present study was to evaluate the potential changes in cancer-specific survival (CSS) after reclassification from fifth to sixth edition TNM. Methods A total of 760 DTC patients managed at our institution from 1961 to 2001 were retrospectively restaged from the fifth to sixth edition TNM. CSS were calculated using Kaplan-Meier method and were compared by the log-rank test. The relative ability of each edition in predicting CSS was calculated by the proportion of variance explained (PVE). Results Upon reclassification, the proportion of T1 and T3 tumors increased from 14.2 to 33.4% and 10.0 to 33.7%; T2 and T4 decreased from 44.2 to 25.0% and 31.6 to 7.9%, respectively; N0 remained unchanged at 66.0%; N1a decreased from 25.7 to 4.7%; N1b increased from 8.4 to 29.3%; stages I and IV tumors increased from 55.7 to 60.3% and 3.4 to 17.6%, respectively; stages II and III tumors decreased from 20.5 to 13.9% and 20.4 to 8.2%, respectively. The sixth edition had a higher PVE value than the fifth edition. Significant differences in CSS were observed between stage III (fifth edition) and stage III (sixth edition) and between stage IV (fifth edition) and stage IVA (sixth edition). Conclusions The sixth edition TNM caused marked changes in the pT, pN and allocation of patients into different tumor stages. It appeared to have superior predictability over the fifth edition.