The clinical and metabolic responses of 12 patients with painful Paget's disease have been assessed before, during and after an 18-day course of treatment with purified porcine calcitonin, 160 MRC units i.m. daily. The indices of response included relief from pain, changes in concentration of serum alkaline phosphatase, calcium, magnesium, electrolytes and plasma inorganic phosphate; balance studies of calcium, phosphate, magnesium and in some cases sodium and potassium were undertaken. Urinary excretion of hydroxyproline was also measured. Remission of pain was complete in 11 of 15 courses of treatment. Coexistent osteoarthrosis was considered to be responsible where remission was incomplete. The mean duration of remission of pain was 12 months. Three patients who received a second course of treatment achieved further remission of pain. Side effects were not troublesome. Serum levels of alkaline phosphatase decreased to 67 per cent of the initial reading and the degree of suppression depended on the initial alkaline phosphatase activity. Urinary hydroxyproline correlated closely with the activity of serum alkaline phosphatase before, during and after treatment. The ratio of serum alkaline phosphatase:hydroxyproline excretion rose at the start and at the termination of treatment. Retention of calcium and inorganic phosphate occurred during the early phase of treatment only. Withdrawal of treatment resulted in a rebound retention of calcium and inorganic phosphate. The degree of calcium retention after treatment correlated with the level of plasma inorganic phosphate prevailing before treatment. The efficacy of these short courses of calcitonin are discussed and the metabolic observations related to theories of bone remodelling at a cellular and hormonal level.