The induction of new blood vessels is critical to the pathogenesis of colon cancer, and inhibition of vascular endothelial growth factor (VEGF) has proven to be an effective approach to the treatment of this malignancy. Another potential therapeutic strategy would utilize endogenous anti-angiogenic molecules such as thrombospondin-1 (TSP-1). However, the regulation of TSP-1 expression in colon cancer is poorly understood. Our results demonstrate that TSP-1 is strongly expressed in normal colonic epithelial cells. However, loss of TSP-1 was observed in early colonic adenomas and it became undetectable in invasive colon cancers. Activation of the Wnt signaling pathway in intestinal epithelial cells repressed TSP-1 gene expression, and inhibition of Wnt signaling in colon cancer cells reversed this repression. Although mutant K-ras also inhibited the TSP-1 promoter in intestinal epithelial cells, silencing of mutant K-ras in colon cancer cells with an activated Wnt pathway did not upregulate TSP-1 expression. Collectively, these findings suggest that activation of the Wnt pathway rather than K-ras plays a more important role in the downregulation of TSP-1 observed in colon cancer.