Summary— Several indirect biochemical indices of bone resorption are increased in patients with carcinoma of the prostate and skeletal metastases. We have examined histological indices of bone resorption in biopsies from affected patients. Bone-forming surfaces and active osteoblast numbers were increased in skeletal sites adjacent to tumour tissue and indices of bone resorption were significantly increased at sites adjacent to tumour. Contrary to expectation, indices of bone resorption were also increased in bone distant from skeletal metastases. These findings suggest that prostatic cancer induces generalised loss of trabecular bone which may minimise disturbances in plasma calcium homeostasis but could contribute to morbidity.