Bone remodeling is a central event in the maintenance of skeletal tissue, and involves cycles of resorption followed by the formation of bone tissue. The activity of osteoclasts and osteoblasts during these cycles is tightly regulated by sys- temic and local factors coupling the action of these cells. Tar- trate-resistant acid phosphatase (TRAP) is predominantly ex- pressed in bone by osteoclasts but has also been detected in osteoblasts and osteocytes. Moreover, TRAP can stimulate the differentiation of mesenchymal lineage cells, i.e. progen- itors of osteoblasts and adipocytes. In order to further ex- plore the effects of TRAP on bone turnover, the structural and molecular phenotypes of osteoclasts and osteoblasts were assessed in TRAP-overexpressing transgenic mice. Transgenic mice of both sexes display increased cortical bone mineral content and density, which cannot be account- ed for by decreased bone resorption since osteoclast num- bers and resorptive activity do not differ from wild-type mice. Examination of the osteoblast phenotype revealed that markers of bone formation, i.e. procollagen type I N- terminal propeptides, and osteoblast lineage markers as well as the TRAP 1B mRNA transcript are increased in TRAP- overexpressing mice. Expression of the osteoclast-selective TRAP 1C mRNA is not increased in TRAP transgenic mice. El- evated expression of TRAP mRNA and protein were detected in osteoblasts, osteocytes and in the bone matrix of TRAP transgenic mice, suggesting that TRAP overexpression in os- teoblast lineage cells is associated with increased cortical bone mineral content and density. The data presented here support the hypothesis that TRAP overexpression in the os- teoblastic cell lineage stimulates the differentiation and/or activation of these cells.