The phenotype of thrombospondin 2 (TSP2)-null mice includes abnormalities in collagen fibrils and increases in ligamentous laxity, vascular density, and bleeding time. In this study, analyses by computerized tomography (CT) revealed that cortical density was increased in long bones of TSP2-null mice. Histomorphometric analysis showed that the mid-diaphyseal endosteal bone formation rate (BFR) of TSP2-null mice was increased in comparison with that of wild-type (WT) animals. Although microgeometric analysis showed that periosteal and endosteal radii were reduced, the mechanical properties of femurs from TSP2-null mice were not significantly different from those of controls, presumably because of the concomitant increase in endosteal bone mass. Bone loss in ovariectomized mice was equivalent for WT and mutant mice, a finding that indicates that TSP2-null animals are capable of normal bone resorption. To further explore the cellular basis for the increased endosteal BFR in TSP2-null mice, marrow stromal cells (MSCs) were isolated and examined in vitro. These cells were found to be present in increased numbers in a colony forming unit (CFU) assay and showed an increased rate of proliferation in vitro. We conclude that TSP2 regulates the proliferation of osteoblast progenitors, directly or indirectly, and that in its absence endosteal bone formation is increased.