Lung cancer, one of the leading causes of death worldwide, is often associated with a state of immune suppression, but the molecular and functional basis remains enigmatic. Evidence is provided in this paper supporting the role of lung cancer-derived soluble lectin, galectin-1, as a culprit in dendritic cell (DC) anergy. We have shown that galectin-1 is highly expressed in lung cancer cell lines, together with the serum and surgical samples from lung cancer patients. Functionally, lung cancer-derived galectin-1 has been shown to alter the phenotypes of monocyte-derived DCs (MdDCs) and impair alloreactive T cell response, concomitant with the increase of CD4(+)CD25(+)FOXP3(+) regulatory T cells. The regulatory effect of galectin-1 is mediated, in part, through its ability to induce, in an Id3 (inhibitor of DNA binding 3)-dependent manner, the expression of IL-10 in monocytes and MdDCs. This effect is inhibited by the addition of lactose, which normalizes the phenotypic and functional alterations seen in MdDCs. Of note, significant upregulation of IL-10 was seen in tumor-infiltrating CD11c(+) DCs in human lung cancer samples. This was also noted in mice transplanted with lung cancer cells, but not in those receiving tumor cells with galectin-1 knockdown. Furthermore, a significant reduction was noted in lung cancer incidence and in the levels of IL-10-expressing, tumor-infiltrating DCs, in mice receiving galectin-1-silenced tumor cells. These results thus suggest that the galectin-1/IL-10 functional axis may be crucial in lung cancer-mediated immune suppression, and that galectin-1 may serve as a target in the development of lung cancer immunotherapy.