Thirty-two patients with osteitis deformans were treated with porcine calcitonin for 240 patient-months. Relief of pain occurred in 10 patients, while a fall in the serum alkaline phosphatase level was observed in 22 out of 24 patients whose treatment lasted for more than two weeks. Pain relief occurred predominantly in patients with pain in the lower limbs, although not all patients with lower limb pain showed improvement. Side effects, usually mild in nature, were reported by half the patients. Antiporcine calcitonin antibodies were detected in sera of six out of 14 patients. The presence of circulating antibodies seemed unrelated to the effect of therapy or to the occurrence of side effects.