B asic fibroblast growth factor (FGF-2), an important modulator of cartilage and bone growth and differentiation, is expressed and regulated in osteoblastic cells. To investigate the role of FGF-2 in bone, we examined mice with a disruption of the Fgf2 gene. Measurement of trabecular bone architecture of the femoral metaphysis of Fgf2^+/+ and Fgf2^–/– adult mice by micro-CT revealed that the platelike trabecular structures were markedly reduced and many of the connecting rods of trabecular bone were lost in the Fgf2^–/– mice. Dynamic histomorphometry confirmed a significant decrease in trabecular bone volume, mineral apposition, and bone formation rates. In addition, there was a profound decreased mineralization of bone marrow stromal cultures from Fgf2^–/– mice. This study provides strong evidence that FGF-2 helps determine bone mass as well as bone formation.