OBJECTIVE:The 9thTNM proposal for thymic epithelial tumours (TETs) introduced size as category in stage I, confirming tumour infiltration type as descriptor for the other stages. Aim of this study is to evaluate the role of tumour size in TETs considering different possible cut-offs also among different subgroups. MATERIAL AND METHODS:Clinical and pathological data of patients from ESTS thymic database who underwent surgery for TETs from1/2000 to 12/2022 were reviewed and analysed. Patients clinical data, tumour characteristics, size and organs infiltration were collected and correlated to overall survival (OS), Disease free Survival (DFS) and Cancer specific Survival (CSS) using Kaplan Meier curves. The log-rank test was used to assess differences between subgroups. A multivariable model was built using Cox-regression analysis including clinical relevant variables resulting significant at univariable (p-value < 0.05). RESULTS:The final analysis was conducted on 2146 patients. Most patients presented tumours size >5 cm(59 %)and without surrounding structures infiltrations (51.3 %). During FUP, a recurrence occurred in 235 (11%) patients, 199 (9.3%) died, 38 due to tumour progression. Multivariable confirmed as independent negative prognostic factors age (p < 0.001), carcinoma/NETT histology (p < 0.001),TETs size >5 cm (p < 0.001, HR 2.16; 95 %CI 1.32-3.50), and infiltration (p = 0.04) for OS; advanced TNM STAGE 9th edition (p < 0.001) and carcinoma/NETT (p = 0.001) for DFS; infiltration presence (p < 0.001) and carcinoma/NETT (p < 0.001) for CSS. Significant differences in OS, DFS and CSS were present considering size cut-off 5 cm (p < 0.001, p < 0.001 and p = 0.001, respectively), while using 3 cm cut-off only DFS (p < 0.001) and CSS (p = 0.001) resulted statistically significant. Significant differences in OS, DFS and CSS were present considering surrounding organ infiltration in TETs ≤ 5 cm (p = 0.004, p < 0.001 and p = 0.001) and in TETs > 5 cm (p = 0.024, p < 0.001 and p < 0.001). CONCLUSIONS:Tumour size resulted a significant prognosticator in TETs, and its associations with infiltration permits to identify different prognostic groups.