A treatment option for early stage breast cancer is nodal evaluation by axillary lymph node dissection (ALND). An alternative to ALND is sentinel lymph node (SLN) biopsy employing radionuclide SLN mapping. This study was designed to investigate the relationship between malignancy spread to the SLN and primary tumor size by reviewing the clinical profile of 20 female breast cancer patients who underwent radionuclide SLN mapping procedure, sentinel lymph node and primary tumor biopsy, as well as axillary lymph node dissection. The accuracy of radionuclide mapping in the identifying the sentinel node and determining the status of the axillary lymph nodes was reviewed. Among the mapped sentinel nodes, 15% were positive for metastatic lymphadenopathy and 85% without malignant spread. The malignant sentinel nodes had a mean size of 5.5 ± 0.87 cm and the negative sentinel nodes had a mean size of 2.95 ±2.2 cm. The SLN identified by radionuclide mapping truly represented the status of the rest of the axillary nodes for the presence or absence of metastatic lymphadenopathy. Primary tumor size is a prognostic factor for cancer spread to the sentinel node. However, the combination of primary tumor histology and tumor size may prove to be a stronger prognostic indicator malignancy spread to the sentinel lymph node.