The bisphosphonates are potent inhibitors of osteoclast-mediated bone resorption and are now the treatment of choice for the management of hypercalcaemia of malignancy. The incidences of hypercalcaemia and other skeletal complications (bone pain, pathological fracture) remain high despite apparent responses to systemic therapy, with particularly high event rates in women with advanced skeletal metastases of breast cancer. This review focuses on studies addressing the long-term efficacy of bisphosphonates to reduce skeletal complications in breast cancer (5 studies) and multiple myeloma (4 studies), with particular reference to controlled studies of sufficient magnitude and duration to allow confidence in the estimation of efficacy. Bearing in mind the limitations of differences in trial design and the lack of direct studies comparing drugs, adequate exposure to a bisphosphonate reduces the incidence of skeletal complication by 30 to 40% in both breast cancer and multiple myeloma. Oral clondronate and intravenous pamidronate have similar efficacy in both diseases, but the duration of efficacy may differ between drugs. Both agents have shown intriguing survival benefits in subgroups of patients. The numbers needed to treat (NNT) to prevent a skeletal complication during one year are lowest in metastatic skeletal disease in breast cancer (NNT < 8) but also compare very favourably with other disease for patients with recurrent nonskeletal breast cancer or multiple myeloma (NNTs 7 to 31 depending on the complication to be prevented). Treatment costs of both breast cancer and multiple myloma are driven by inpatient and outpatient hospital visits so that bisphosphonate regimens should be developed that reduce both. Further research is required to determine if subgroups of patients can be better identified that will derive particular benefit, or perhaps no benefit at all, from bisphosphonate therapy. It is not known whether more potent bisphosphonates will deliver greater clinical efficacy in the future.