Removal of the androgen receptor (AR) from bone-forming cells has been shown to reduce trabecular bone volume in male mice. In female mice, the role of AR in the regulation of bone homeostasis has been poorly understood. We generated a mouse strain in which the AR is completely inactivated only in mineralizing osteoblasts and osteocytes by breeding mice carrying osteocalcin promoter-regulated Cre-recombinase with mice possessing loxP recombination sites flanking exon 2 of the AR gene (AR ΔOB/ΔOB mice). In female AR ΔOB/ΔOB mice, the trabecular bone volume was reduced owing to a smaller number of trabeculae at 6 months of age compared with the control AR fl/fl animals. In male AR ΔOB/ΔOB mice, an increase in trabecular bone separation could already be detected at 3.5 months of age, and at 6 months, the trabecular bone volume was significantly reduced compared with that of male AR fl/fl mice. No AR-dependent changes were observed in the cortical bone of either sex. On the basis of micro-computed tomography and histomorphometry, we conclude that in male mice, the AR is involved in the regulation of osteoclast number by osteoblasts, whereas in female mice, the lack of the AR in the bone-forming cells leads to a decreased number of trabeculae upon aging.