Tumor angiogenesis depends on the vascular endothelial growth factor (VEGF), which exists in multiple splicing isoforms, including the most abundant VEGF165 and VEGF121. We have previously shown that the differential capacity of these two VEGF isoforms to bind Neuropilin-1 accounts for their diverse ability to recruit Nrp1-expressing monocytes (NEMs), resulting in a different arteriogenic potential. Here we measure the expression of VEGF165 and VEGF121 in human cancer and their influence on tumor growth and vascularization. We measured the expression levels of VEGF165 and VEGF121 in human colorectal cancer and found that VEGF121 was more expressed than VEGF165, particularly in patients with extensive lymph node infiltration. Overexpressing either VEGF165 or VEGF121 in a cancer mouse model, we observed that the former decreased, whereas the latter increased tumor growth. In both clinical and experimental tumors, VEGF165 expression resulted in the recruitment of NEMs, paralleled by maturation of the tumor vascular network. Finally, hypoxia induced a shift toward the VEGF165 isoform in the central core of human cancers, as well as in various types of cultured cells. These results demonstrate that the two VEGF splicing isoforms are differentially expressed in colorectal cancers, exerting opposite effects on tumor growth and vessel maturation.