Embryonic stem(ES) cells are immature cells that have the ability of self-renewal, and they give rise to neurons, astrocytes and oligodendrocytes. In order to investigate their ability to improve cognitive function in an animal model of stroke, we examined the effect of ES cell transplantation on the ischemia-induced impairments of learning and memory that were observed in the Morris water maze for rats. Human embryonic stem(hES) cells(Mb03, as registered in the NIH) established from frozen-thawed blastocysts were partially differentiated according to the 4-/4+ protocol using retinoic acid. The formed embryoid bodies were dispersed by trypsin and a total of 3x104 cells were grafted into the hippocampus(CA1) 2 weeks after ischemia. Implantation of the hES cells produced significant reversals of the ischemia-induced deficit in learning and memory on the spatial learning task. These results demonstrated that grafted hES cells improved spatial recognition, and they may have valuable therapeutic potential as a treatment for stroke.