Sentinel lymph node (SLN) biopsies using radioactive tracers have been reported to detect the metastatic status of the axillary lymph nodes in breast cancer patients. However, the optimal radioactive tracer particle sizes remain to be determined. In this study, identification of SLNs with large radiolabeled tin colloid particles was evaluated. Seventy-five patients with T1-2, N0 breast cancer were enrolled. Two hours prior to surgery, 1 to 2.5 mL technetium-99m-labeled tin colloid particles were injected around the tumor under ultrasonographic guidance. Immediately before the operation, dye fluids were also injected around the tumor to increase the interstitial pressure. After axillary dissection, lymph nodes with x100 or more uptake of radioactivity than the mean of the other lymph nodes are considered to be SLN. All lymph nodes from the axillary dissections were pathologically investigated, and the characteristics of SLNs were evaluated. SLNs were clearly identified in 74 of 75 patients (98.7%). Of 37 patients without SLN metastasis, pathological investigation revealed no further involvement of the remaining non-SLNs. The SLNs tended to be larger in size, and more than 50% were located in the lower medial site of the axilla. This is the first study to show that SLNs could successfully be identified with radiolabeled tin colloid particles. When SLNs were negative for metastases, non-SLNs were always negative.