We herein describe a case of hypercalcemic crisis in a 52-year-old Japanese woman. She suffered from thirst and fatigue for one month. Her serum calcium (a) levels were 19.0 mg/dL, and she was diagnosed with hypercalcemic crisis. Circulating levels of parathyroid and thyroid hormones were elevated. She was diagnosed with primary hyperparathyroidism accompanied by Graves' disease. Thyroparathyroidectomy was performed after circulating levels of Ca and thyroid hormones were normalized. Both of primary hyperparathyroidism and Graves' disease can contribute and accelerate hypercalcemia, resulting in a state of crisis. The possibility of their coexistence should therefore be taken into consideration in cases of hypercalcemia.