• Hypomagnesemia (serum megnesium concentration, <1.5 mg/dL [0.62 mmol/L]) and hypocalcemia (serum calcium concentration, <8 mg/dL [2.00 mmol/L]) have been reported in polycythemic infants, as well as in infants of diabetic mothers (IDMs). These latter infants are at risk for neonatal polycythemia (venous hematocrit, ≤65% [0.65]). We tested the hypothesis that neonatal polycythemia in IDMs is associated with increased serum calcitonin concentration, hypomagnesemia, and hypocalcemia. Serum magnesium and calcium concentrations were measured at 24 and 72 hours of age in 76 IDMs; serum calcitonin concentration was measured at 24 hours of age. Peripheral venous spun hematocrit was measured between 2 and 4 hours of age. The rates of hypomagnesemia and hypocalcemia were similar in polycythemic and nonpolycythemic IDMs (0% vs 9% and 56% vs 49%, respectively). The serum calcitonin concentration was similar in both groups. There was no correlation between hematocrit and the serum magnesium or calcium concentration; a significant association existed between hypocalcemia and hypomagnesemia.