Introduction The ketogenic diet is increasingly used in management of pediatric refractory epilepsy and significantly affects the bone mineral metabolism. Although hypercalciuria and loss of bone mineral density are common in patients on the ketogenic diet, there is a few report associated ketogenic diet-induced hypercalcemia. We describe one patient who developed severe hypercalcemia 10 months after commencing the ketogenic diet. Case The case was a 21-month-old girl with multifocal polymicrogyria and epileptic spasms refractory to therapy with levetiracetam, vigabatrin, and topiramate. She started 2.5:1 ketogenic diet at 11 months of age and achieved freedom of spasms. Her baseline calcium level was 9.4 mg/dL before starting the diet. Ten months after commencing the ketogenic diet, she presented to the emergency room with generalized weakness and respiratory instability. Laboratory work-up showed high level of serum calcium (14.4 mg/dL) with ionized calcium of 5.49 mg/dL, low levels of PTH (3.0 pg/mL), normal level of 1,25(OH)2D3 (19.9 pg/mL) and elevated urinary calcium: creatinine ratio (599.8 mg/g). Concurrent hypomagnesemia, which might be secondary to hypercalcemia, was also observed with serum magnesium of 1.15 mg/dL. Her serum calcium and magnesium level were almost normalized on treatment with saline hydration, furosemide and magnesium replacement. Despite of lowering the ratio of ketogenic diet to 2:1, hypercalcemia persisted after discharge and her serum calcium and magnesium concentration were completely normalized within 1 week of discontinuation of ketogenic diet without recurrence of spasms. Conclusion Although hypercalcemia is an uncommon complication of the ketogenic diet, it might lead to potentially life-threatening condition during diet therapy. This case highlights the need for close surveillance of hypercalcemia in patients with ketogenic diet and further research clarifying the association between ketogenic diet and hypercalcemia.