It has been suggested that hypercalcitoninemia may contribute to neonatal hypocalcemia in infants of diabetic mothers (IDM). Because the role of calcitonin (CT) in Ca metabolism in humans is questionable, we hypothesized that serum CT peaks similarly after birth in IDM and controls and that serum Ca concentrations do not correlate with serum CT. Forty-seven term IDM (White classes B-RT) were compared with 31 controls. Controls were born after normal pregnancies, labors, and deliveries. Blood samples (cord and 24 h) were analyzed for Ca, Mg, parathyroid hormone (PTH), and CT. Repeated measures analysis showed increasing serum Mg, PTH, and CT, and decreasing Ca over time. The incidence of hypocalcemia was significantly higher in the diabetic group (p < 0.01) and the incidence of hypomagnesemia was borderline significantly higher (p < 0.06). There were no differences in cord or 24-h serum concentrations of CT between groups. In multiple regression analysis, serum Ca and PTH were correlated (p < 0.02, R2= 0.33), but not serum Ca and CT; the increase in serum PTH in relation to serum Ca at the nadir (24 h) correlated directly with serum Mg concentrations (R2= 0.31, p < 0.05). Thus, serum CT increases after birth irrespective of the rate of decrease of serum Ca in both IDM and controls; high CT concentrations observed after birth (as compared with adult norms) do not seem to play a role in the pathogenesis of neonatal hypocalcemia in IDM; and responsiveness of parathyroid gland at birth is adversely affected by hypomagnesemia, which supports the theory of functional hypoparathyroidism in Mg deficiency.