As macrophages are some of the first cells to encounter metastatic tumor cells in sentinel lymph nodes (SLN) and natural killer (NK) cells are critical to the cytotoxicity of abnormal cells, we sought to determine if these cell populations were altered in the presence of nodal metastasis. We used immunohistochemistry to assess the SLN of 47 patients with breast cancer (36 with nodal metastasis and 11 without nodal metastasis) and 10 control lymph nodes. We assessed metastatic areas and nonmetastatic areas separately for CD163, a marker of macrophages, and ANK-1, a marker for precursors of activated NK cells. Positively stained cells were manually counted in multiple high-power fields and averaged. Groups were compared with the Kruskal-Wallis test. Spearman rank order test was used for correlations. There was a lower frequency of CD163(+) macrophages in the SLN of patients with breast cancer (median, 11.0 %; range, 4.1-20.4 %) than controls (median, 16.5 %; range, 8.9-19.6 %; p = 0.002). There were no differences in the expression of ANK between patients with cancer (median, 1.4 %; range, 0.23-6.3 %) and controls (median, 1.5 %; range, 0.60-5.4 %; p = 0.5). In patients with nodal metastasis, the accumulation of CD163(+) cells in the sinuses correlated negatively with CD8(+) tumor-infiltrating lymphocytes (r (2) = 0.23; p = 0.001). These results suggest that the reduction of CD163(+) macrophages in the sinuses of the SLN is associated with nodal metastasis and may have a role in regional immunity.