INTRODUCTION: The tall cell subtype of papillary thyroid carcinoma (PTC) is considered to have poor prognosis, but its diagnostic criteria have been modified on several occasions. The aim of our study was to find the best diagnostic criteria to relate the tall cell subtype with aggressiveness and poor prognosis. MATERIAL AND METHODS: In 171 cases of classic subtype and tall cell subtype of PTC, the percentage of cells between two and three times taller than wide (C2-3) and cells more than three times taller than wide (Cx3) was determined by two observers, after obtaining reference images. Groups were made based on the percentage of cells of each component (<
2%, between 2 and 10%, between 11 and 30%, between 31 and 50% and >
50%) and were correlated with clinical and histopathological data. Interobserver agreement was also assessed. RESULTS: According to the current WHO classification criteria, only 8 cases (5%) of tall cell subtype were identified. The comparison of the risk of recurrence at 5 years based on the percentage and type of cells showed that it increased significantly from >
10% of the Cx3 cell component, while there were no significant differences in recurrence rate with any C2-3 cell percentages. Very good interobserver agreement was obtained (Kappa index: 0.83). CONCLUSIONS: We consider relevant reporting the presence of Cx3 cells from 10% in PTC as it is associated with a significant increase in the recurrence rate at 5 years. Interobserver agreement is very good if reference images are available.