To test the hypothesis that calcitonin (CT) deficiency may contribute to bone mineral loss in insulin-dependent diabetes mellitus (IDDM), we studied basal and calcium stimulated (2 mg/kg body wt. in 5 min) CT levels in 15 children with IDDM and osteopenia. Ten age-sex matched healthy children were studied as controls. Since extractable CT (exCT) allows more sensitive and specific measurement of CT monomer, we measured both total serum CT (tCT) and exCT. Diabetic children had slightly but significantly (P less than 0.05) higher basal levels of both tCT (24.5 +/- 7.1 ng/l) and exCT (5.6 +/- 1.6 ng/l) than controls (tCT: 18.7 +/- 5.4 ng/l; exCT: 4.3 +/- 1.2 ng/l). Calcium stimulation test pointed out significant increase (P less than 0.001) of tCT and exCT in both groups with peak values not significantly different in IDDM in respect to controls. However, diabetic children showed a reduced CT reserve evidenced by a lower peak/basal ratio (diabetics: tCT 1.68, exCT 1.84; controls: tCT 2.49, exCT 2.88) and by a more rapid decrease in CT levels. We conclude that CT deficiency is not a causative factor of diabetic osteopenia. The slightly higher basal CT values suggest that an increased bone reabsorption may be operative in IDDM and it stimulates CT secretion. This chronic "C" cell stimulation may induce the reduction in CT reserve observed employing the calcium infusion test.