This study has examined the effect of prostate cancer on bone matrix formation and mineralisation by osteoblasts, with special reference to osteomalacia. Sixty-seven patients with prostatic bone metastases underwent transiliac bone biopsy after double tetracycline labelling. Histomorphometric analysis was then undertaken in areas distant from, local to and infiltrated by prostate cancer. In bone free of tumour (n = 45) and bone surrounding metastases (n = 7) both matrix formation and corrected mineral apposition rate were low. By comparison osteoid surface, osteoid volume and mineral apposition rate were markedly increased within metastases (tumor-free bone vs. metastatic bone, p < 0.0001), a finding consistent with a high bone turnover state. Although osteoblast function was disturbed both within metastases and in tumour-free areas, classical osteomalacia was not associated with prostate cancer.