The effects of mithramycin, an antibiotic with cytotoxic activity, were observed in 15 patients with active Paget's disease of bone. Mithramycin is shown to have a consistent effect in suppressing the activity of Paget's disease, as reflected by changes in serum alkaline phosphatase and urinary hydroxyproline levels, and the occurrence of symptomatic relief. These responses are probably mediated by a cytotoxic effect on osteoclasts. Several patients have shown prolonged remissions. This agent shows considerable promise as an effective treatment for Paget's disease of bone but means of enhancing the therapeutic:toxicity ratio need further investigation.