The hypothesis that Paget's disease of bone is a low grade neoplastic process led us to use the cytotoxic antibiotic mithramycin in its treatment. The dramatic effects observed on the serum calcium and alkaline phosphatase, and urinary hydroxyproline are compatible with the concept that mithramycin is primarily toxic to osteoclasts. Subjective and objective clinical effects establish this agent as useful in the treatment of Paget's disease despite its observed toxicity to other organ systems.