Background: Breast cancer is the most common cancer among women worldwide. The aim of this study wasto investigate the relationship between tumor size and axillary lymph node involvement (ALNI) in patients withinvasive lesions, to find the best candidates for a full axillary dissection. Additionally, we evaluated the associationbetween tumor size and invasive behavior. The study was based on data from 789 patients with histopathologicallyproven invasive breast cancer diagnosed in Shohada University hospital in Tehran, Iran (1993-2009). Cinicaland histopathological characteristics of tumors were collected. Patients were divided into 6 groups accordingto primary tumor size: group I (0.1-≤1cm), II (1.1-≤2cm), III (2.1-≤3cm), IV (3.1-≤4cm), V (4.1-≤5cm) andVI (>5cm). The mean(±SD) size of primary tumor at the time of diagnosis was 3.59±2.69 cm that graduallydeclined during the course of study. There was a significant correlation between tumor size and ALNI (p<0.001).A significant positive correlation between primary tumor size and involvement of surrounding tissue was alsofound (p<0.001). The mean number of LNI in group VI was significantly higher than other groups (p<0.05).Weobserved more involvement of lymph nodes, blood vessels, skin and areola-nipple tissue with increase in tumorsize.We found 15.3% overall incidence of ALNI in tumors ≤2 cm, indicating the need for more investigation toomit full axillary lymph node dissection with an acceptable risk for tumors below this diameter. While in patientswith tumors ≥2 cm, 84.3% of them had nodal metastases, so the best management for this group would be a fullALND. Tumor size is a significant predictor of ALNM and involvement of surrounding tissue, so that an exactestimation of the size of primary tumor is necessary prior to surgery to make the best decision for managementof patients with invasive breast cancer.