Osteoporosis is a common finding in Turner's syndrome. To test the hypothesis that calcitonin deficiency may contribute to bone mineral loss in Turner's syndrome, we studied basal and calcium-stimulated (2 mg/kg body weight in 5 min) levels of total calcitonin, extractable calcitonin and katacalcin in 15 girls with Turner's syndrome and osteoporosis. Fifteen age-matched healthy girls were studied as controls. Both basal calcitonin (total and extractable) and katacalcin values were not significantly different in patients with Turner's syndrome in comparison with those of the controls. The calcium stimulation test showed a similar "C" cell secretory reserve in both groups. The calculation of delta CT/delta iCa of total and extractable calcitonin and delta KC/delta iCa, which accounts for individual variations in serum ionized calcium increases, did not show any significant difference between girls with Turner's syndrome and controls. We conclude that calcitonin deficiency is not a causative factor of osteoporosis in girls with Turner's syndrome and that in this syndrome long-life estrogen deficiency does not impair "C" cell secretory activity.