Tumours which involve the skeleton do so by producing humoral factors which increase osteoclast and osteoblast activity. Increases in osteoclast activity lead to osteolytic bone destruction and sometimes to hypercalcaemia. Osteolytic metastases are common, and are found most often in patients with lung and breast cancer and in myeloma. The tumour-associated factors responsible are multiple and probably different in each case. Osteoblastic metastases occur most frequently in metastatic cancer of the prostate, and are due to osteoblast stimulating factors released by the tumour cells which have not, as yet, been identified. Agents such as bisphosphonates which inhibit osteoclastic bone resorption are useful in the prevention and treatment of patients with osteolytic metastases, although the precise mechanisms by which these agents work are not yet understood.