The adult bone marrow (BM) has been generally considered to be composed of hematopoietic tissue and the associated supporting stroma. There is accumulating evidence that a subset of multipotential cells exists within the latter compartment, commonly referred to as mesenchymal stem cells (MSCs). These cells can be easily expanded ex vivo and induced to differentiate into skeletal connective tissues, thereby emerging as attractive candidates for various therapeutic applications. Despite the extensive in vitro characterization of MSCs, at present, little is known about their in vivo properties or anatomical location. Here we review the efficacy of the different surface markers used to isolate BM MSCs and highlight current data suggesting that BM MSCs in situ are associated with vascular sinus walls and probably belong to the family of vascular smooth muscle cells.