In the present study, we tested the hypothesis that resident progenitor cells may contribute to tumor vascularization and growth. CD133⁺ cells were isolated from 30 human renal carcinomas and characterized as renal resident progenitor cells on the basis of the expression of renal embryonic and mesenchymal stem cell markers. CD133⁺ progenitors differentiated into endothelial and epithelial cells as the normal CD133⁺ counterpart present in renal tissue. In the presence of tumor-derived growth factors, these cells were committed to differentiate into endothelial cells able to form vessels in vivo in SCID mice. Undifferentiated CD133⁺ progenitors were unable to form tumors when transplanted alone in SCID mice. When co-transplanted with renal carcinoma cells, CD133⁺ progenitors significantly enhanced tumor development and growth. This effect was not attributable to the tumorigenic nature of CD133⁺ progenitor cells because the same results were obtained with CD133⁺ cells from normal kidney. CD133⁺ progenitors contributed to tumor vascularization as the majority of neoformed vessels present within the transplanted tumors were of human origin and derived from the co-transplanted CD133⁺ progenitors. In conclusion, these results indicate the presence of a renal progenitor cell population in renal carcinomas that may differentiate in endothelial cells and favor vascularization and tumor growth.