Localization of metastases within the sentinel lymph nodes (SLNs) of breast cancer has not been studied. Forty SLNs from 36 patients with operable primary breast cancers were identified by means of lymphatic mapping with patent blue dye. The junction between the patent blue-stained lymphatic vessel draining the tumor and the SLN was labeled with alcian blue. Metastases within the serially sectioned SLNs were assigned to the alcian blue-labeled side, to the opposite side of the virtually halved nodes, or both. Eight SLNs were negative for metastasis. Eleven SLNs had metastases only in the blue half. Only 4 cases had larger metastases in the nonblue half. Metastases are more likely to be located in the vicinity of the inflow junction of the identifiable lymphatic draining the tumor and the SLN. This should be considered when SLNs are examined, especially when they are halved for different studies.