Purpose: Impaired immune functions in the sentinel lymph node (SLN) may facilitate early metastatic events during melanoma development. Local potentiation of tumor-specific T cell reactivity may be a valuable adjuvant treatment option. Experimental Design: We examined the effect of locally administered granulocyte/macrophage-colony stimulating factor (GM-CSF) on the frequency of tumor-specific CD8+ T cells in the SLN and blood of patients with stage I melanoma. Twelve patients were randomly assigned to preoperative local administration of either recombinant human GM-CSF or NaCl 0.9%. CD8+ T cells from SLN and peripheral blood were tested for reactivity in an IFNγ ELISPOT assay against the full-length MART-1 antigen and a number of HLA-A1, HLA-A2, and HLA-A3-restricted epitopes derived from a range of melanoma-associated antigens. Results: Melanoma-specific CD8+ T cell response rates in the SLN were one of six for the control group and four of six for the GM-CSF-administered group. Only one patient had detectable tumor-specific CD8+ T cells in the blood, but at lower frequencies than in the SLN. All patients with detectable tumor-specific CD8+ T cells had a percentage of CD1a+ SLN-dendritic cells (DC) above the median (i.e., 0.33%). This association between above median CD1a+ SLN-DC frequencies and tumor antigen-specific CD8+ T cell reactivity was significant in a two-sided Fisher's exact test (P = 0.015). Conclusions: Locally primed antitumor T cell responses in the SLN are detectable as early as stage I of melanoma development and may be enhanced by GM-CSF-induced increases in SLN-DC frequencies.