The cellular mechanism through which osseous breast cancer metastases induce the focal destruction of bone (tumor osteolysis) is unknown. An athymic mouse model designed for the study of tumor osteolysis was developed and the influence of two human breast cancer tumors on bone was studied. Tumor-induced osteolysis occurred between 7 and 10 weeks after inoculation of mouse femora with MDA-MB-231 or MDA-MB-435s breast cancer cells. An increase in osteoclast number and an increase in osteoclast size (area) were detected when tumor-bearing and sham-injected limbs were compared. In vitro analysis of the influence of the tumor-conditioned medium on osteoclast-mediated bone resorption revealed that this conditioned medium stimulated the resorption by increasing both the number of osteoclasts bound to bone and the number of bone resorption pits formed per osteoclast. In addition, in vitro analysis of the influence of breast cancer tumor cells on osteoclast formation or survival, or both, demonstrated that breast cancer cells induced a dramatic increase in the number of osteoclasts detected in culture. Taken in total these findings suggest that human breast cancer tumors induce osteolysis by enhancing osteoclast adherence to bone, stimulating osteoclast-mediated bone resorption and either prolonging the survival of osteoclasts or increasing osteoclast formation.