The introduction of sentinel node biopsy (SNB) has led to a significant decrease of axillary lymph node dissections (ALND). The importance of the extracapsular extension (ECE) in the sentinel lymph node (SN) remains unclear. The data of 635 patients with T1-T2N0M0 invasive breast cancer who underwent SNB between 2014 and 2018 were retrospectively analysed. 25% of the SNB patients (158) had metastasis in the SNs. These patients were grouped based on the presence or absence of ECE. The main objective of our study was to analyse the occurrence of massive (>3) node metastasis in the case of ECE negative and ECE positive patients, where ALND was performed. There were 91/158 patients (58%) in the ECE negative group and 67/158 patients (42%) in the ECE positive group. ALND was performed in 42% of the ECE negative and in 69% of the ECE positive patients. There were no significant differences in the mean age of the patients; size, histological type and grade of the tumours, presence of lymphovascular invasion and proportion of hormone and HER2 receptor positivities. In the ECE negative ALND group, pN1 involvement was 82%, pN2+pN3 involvement represented 18% of cases. In the ECE positive ALND group, pN1 involvement was 60%, pN2+pN3 involvement was found in 40% of cases. The presence of ECE was associated with greater axillary disease burden. These results show a significant difference ( = 0.038). ECE of the SN is an important predictor for non-sentinel lymph node involvement. These data suggest, when ECE is confirmed, it is a further factor to be considered in deciding about ALND.