The effectiveness of calcitonin on the vertebral fracture rate in postmenopausal osteoporosis was assessed through the skeletal deformity index (SDI) and the new vertebral fracture rate per 100 patient-years in a group of 32 women with postmenopausal osteoporosis treated by us with 100 IU of salmon calcitonin and 500 mg of elemental calcium for 10 consecutive days each month, and in another group of 28 women with postmenopausal osteoporosis treated with 500 mg of elemental calcium only for 10 consecutive days each month. Both groups were age-matched. The follow-up was a retrospective randomized study over 24 months. Thirty of the 32 women of the calcitonin group and 27 of 28 women of the calcium group finished treatment. The SDI was stabilized after six months in the calcitonin group (0.57 +/- 0.13, 0.62 +/- 0.18, 0.63 +/- 0.16 and 0.64 +/- 0.17, at base line, 6, 12 and 24 months respectively). The calcium group showed a significant increase only at 12 months (P less than 0.01) and 24 months (P less than 0.05) (0.61 +/- 0.16, 0.63 +/- 0.16, 0.69 +/- 0.16, and 0.73 +/- 0.15, at base line, 6, 12 and 24 months respectively). At 24 months, the new vertebral fracture rate decreased by 60% (20%, 14% and 8% at 6, 12 and 24 months respectively) in the calcitonin group and increased by 35% (31%, 33% and 42%, at 6, 12 and 24 months respectively) in the calcium group (P less than 0.025). These results show that calcitonin induced a significant reduction in postmenopausal osteoporotic vertebral fractures.