Bone marrow progenitor cells have been shown to contribute to a small proportion of cells in nonhematopoietic tissues including the brain. In the acute unilateral middle cerebral artery occlusion model in spontaneously hypertensive rats following male-to-female bone marrow transplantation, we present data suggesting that 55% more marrow-derived cells, in general, and 161% more GFAP-positive astrocytes, in particular, migrate preferentially to the ischemic cortex than to the contralateral non-ischemic hemisphere. In addition to their biological significance, our findings could have therapeutic implications. Marrow-derived progenitor cells could potentially be used as vehicles for ex vivo gene transfer to the brain.