The TNM classification of UICC is used for predicting the outcome of thyroid cancer. The 6th edition changed the description of primary tumor (T), regional lymph node (N) and the staging group. The aim of this study was to compare the ability of the 5th and the 6th editions to predict outcome. The two classifications were applied in a retrospective analysis of 90 patients from HUCFF. Sixty-nine patients had papillary carcinoma, 14 follicular, 4 Hürthle cell, and 3 mixed. Patients were followed for a mean period of 58.3 months. At the end of follow-up, 49 patients were disease-free, 23 persisted with disease, 4 had cervical recurrence, 11 had metastases and 3 died. According to the 6th edition, 19 patients were classified as T1, compared to 7 based on the 5th edition; 19 patients were T2 compared to 30; 14 were T3 compared to 10; 22 were T4 compared to 27, and 16 patients were Tx. Both editions showed comparable remissions for stages I, II, and III. For the stage IV there was a significant change in remission, however there was no difference comparing IV and IV C.