High aldehyde dehydrogenase activity has been proved useful for the identification of a small subpopulation of cancer stem cells (CSCs) that are responsible for tumor maintenance and relapse. However, the identification of renal CSCs based on high aldehyde dehydrogenase activity is still absent. By using a xenograft model in which cells from renal carcinoma cell line ACHN and Caki-2 were grown in NOD/SCID mice, we have identified a highly tumorigenic renal CSCs based on high aldehyde dehydrogenase activity. Isolated cancer cells with relatively high aldehyde dehydrogenase activity (ALDHbr cells) displayed several stem cell properties in vitro, including capacities for proliferation, clonogenic ablility, self-renewal, and increased expression of stem cell marker genes Oct3/4A,Nanog and Pax-2. In vivo experiments showed that ALDHbr cells were enriched with greater tumorigenicity compared to their counterparts with low ALDH activity (ALDHlo cells), generating new tumors with as few as 25 cells. The isolation of renal CSCs by their high ALDH activity may lead to a new insight into the study of renal tumor-initiating cells, and may lead to effective prognostic and therapeutic strategies for the treatment of kidney cancer patients.