A series of 78 patients with metastatic bone disease from prostate cancer underwent iliac crest biopsy, enabling histomorphometric quantification of eroded bone surface and bone volume in both tumour-free and metastatic bone tissue. Eroded surfaces in tumour-free specimens were high in patients with active compared to stable disease but bone volume was maintained in both groups, whilst in bone surrounding micrometastases (n = 8) eroded surfaces were further increased and bone volume reduced. Eroded surfaces within metastases were greater still but were associated with increased bone volume due to replacement of the existing trabecular tissue with abnormal woven bone, giving an overall appearance of sclerosis. These results show that the effect of prostate cancer on bone tissue is complex, involving differential disturbance of bone formation and resorption within metastases, in bone surrounding tumour invasion and in the tumour-free skeleton.