Background: The tendency to spread to sentinel lymph node (SLN) may differ depending on the biological, clinical, and histopathological features of tumors. If the factors that affect SLN metastasis (SLNM) are known, there may be no need to perform SLN biopsy (SLNB) in some groups. Objectives: This study aimed to investigate the factors affecting SLNM in patients who underwent surgery and SLNB before (surgery group) or after (neoadjuvant chemotherapy group) systemic therapy in the light of current biological characteristics of tumors and patients. Methods: The study included patients who were operated on for breast cancer and underwent SLNB in our institute between 2017 and 2019. The study included a total of 1, 050 patients, who were divided into the surgery (n=900) and neoadjuvant chemotherapy (NAC) groups (n=150). The patients' tumor localization, tumor size, histological subtype, grade, receptor status, lymphovascular invasion (LVI) status, the number of sentinel lymph nodes removed, metastatic lymph nodes in SLNB, and axillary dissection status were analyzed in this study. Results: The study included a total of 1, 050 patients, who were assigned to the surgery (n=900) and NAC groups (n=150). Of the patients, 311 (34. 5%) cases had SLNM. In the surgery group, multivariate analyses showed that grade III, LVI, Her2 (+) increased the risk of metastasis. In the NAC group, the analyses showed Pre-NAC clinical findings of LN metastasis and luminal A subtypes as effective factors. The factors affecting SLNM were analyzed, and the univariate analyses showed that grades II and III, a tumor size of>2 cm, LVI, Her2 (+), and triple negative increased the risk of metastasis. The analyses also revealed LVI as the most important risk factor for SLN metastasis. Conclusion: Knowing the factors affecting SLNM can provide clues for the type of intervention, reconstruction, and radiotherapy planning of patients to be operated on directly or after NAC. In our study, it was found that patient age, tumor size, tumor biology, tumor grade, and especially LVI status were very important in predicting SLN positivity. It is believed that these features should be taken into account when determining the treatment strategy.