AbstractBreast cancer is frequently associated with osteolytic bone metastasis, where osteoclasts play a major role in bone destruction. Recently, osteoclast differentiation factor (RANKL) has been identified as a prerequisite for the formation and maintenance of osteoclasts from haematopoietic precursors. To elucidate the mechanism of osteoclastogenesis and bone destruction in bone‐residing breast cancer, PTHrP‐producing (MCF‐7) and ‐non‐producing (MCF‐7UP) human breast cancer cells were subcutaneously injected into the forehead of nude mice maintained without oestrogen supplement. One, two, and three weeks thereafter, the expression of RANKL and PTHrP mRNA, and osteoclastogenesis were analysed by in situ hybridization and TRAP staining. In MCF‐7 cells, at early stages, spindle‐shaped stromal cells and osteoblasts on the bone surface expressed RANKL, then numerous osteoclasts were induced on the periosteal bone surface. Three weeks after the transplantation, MCF‐7 cancer cells migrated onto the eroded bone surface, where they survived apoptosis. At all stages, RANKL expression was confined to the stromal/osteoblastic cells, whereas PTHrP was confined to the MCF‐7 breast cancer cells. On the other hand, PTHrP was negative in MCF‐7UP cells at all stages, and neither induction of osteoclasts nor infiltrative growth of cancer cells was observed. Moreover, in vitro treatment with PTHrP resulted in increased RANKL mRNA expression and transcription activity in the MC3T3‐E1 mouse osteoblastic cell line. Thus PTHrP induces osteoclastic bone resorption through the transactivation of the RANKL gene on stromal/osteoblastic cells, affording a bone microenvironment conducive to the survival of PTHrP‐producing cancer cells. Copyright © 2002 John Wiley & Sons, Ltd.