Mutations that cause constitutive activation of fibroblast growth factor receptor 3 (FGFR3) result in skeletal disorders that are characterized by short-limbed dwarfism and premature closure of cranial sutures. In previous work, it was shown that congenital deficiency of FGFR3 led to skeletal overgrowth. Using a combination of imaging, classic histology and molecular cell biology we now show that young adult FGFR3(-/-) mice are osteopenic due to reduced cortical bone thickness and defective trabecular bone mineralization. The reduction in mineralized bone and lack of trabecular connectivity observed by micro-computed tomography were confirmed in histological and histomorphometric analyses, which revealed a significant decrease in calcein labelling of mineralizing surfaces and a significant increase in osteoid in the long bones of 4-month-old FGFR3(-/-) mice. These alterations were associated with increased staining for recognized markers of differentiated osteoblasts and increased numbers of tartrate-resistant acid phsophatase postitive osteoclasts. Primary cultures of adherent bone marrow-derived cells from FGFR3(-/-) mice expressed markers of differentiated osteoblasts but developed fewer mineralized nodules than FGFR3(+/+) cultures of the same age. Our observations reveal a role for FGFR3 in post-natal bone growth and remodelling, which identifies it as a potential therapeutic target for osteopenic disorders and those associated with defective bone mineralization.