Altered mental status in a child is a potentially life-threatening condition with a broad differential including vascular, toxin-mediated, infectious, metabolic, and traumatic causes. Hypercalcemia is a rare cause of altered mental status in children. We report a case of a 13-month-old boy with familial hypocalciuric hypercalcemia who presented to our emergency department with altered mental status and hematemesis. Familial hypocalciuric hypercalcemia is a rare cause of hypercalcemia that usually presents with asymptomatic hypercalcemia. This case illustrates the presentation of severe hypercalcemia and reviews the initial management and evaluation of hypercalcemia in children.