© 2015 by De Gruyter. Background: Although most hypocalcemia with hypomagenesemia in the neonatal period is due to transient neonatal hypoparathyroidism, magnesium channel defects should also be considered. Case: We report a case of persistent hypomagnesemia in an 8-day-old Hispanic male who presented with generalized seizures. He was initially found to have hypomagnesemia, hypocalcemia, hyperphosphatemia and normal parathyroid hormone. Serum calcium normalized with administration of calcitriol and calcium carbonate. Serum magnesium improved with oral magnesium sulfate. However, 1 week after magnesium was discontinued, serum magnesium declined to 0.5 mg/dL. Magnesium supplementation was immediately restarted, and periodic seizure activity resolved after serum magnesium concentration was maintained above 0.9 mg/dL. The child was eventually weaned off oral calcium and calcitriol with persistent normocalemia. However, supraphysiologic oral magnesium doses were necessary to prevent seizures and maintain serum magnesium at the low limit of normal. Methods and results: As his clinical presentation suggested primary renal magnesium wastage, TRPM6 gene mutations were suspected; subsequent genetic testing revealed the child to be compound heterozygous for TRPM6 mutations. Conclusion: Two novel TRPM6 mutations are described with a new geographic and ethnic origin. This case highlights the importance of recognizing disorders of magnesium imbalance and describing new genetic mutations.