A 41-year-old woman was referred to our hospital with complaints of general fatigue, appetite loss, nausea and vomiting. Blood examination revealed high serum calcium level (21.6 mg/dl) and high serum parathyroid hormone level. Although enlarged parathyroid glands could not be clearly detected via ultrasonography, computed tomography and scintigraphy, we diagnosed her with hypercalcemic crisis due to primary hyperparathyroidism and performed parathyroidectomy after conservative therapy. A parathyroid tumor measuring 1.9 x 1.1 x 1.0 cm, and weighing 1,100 mg was found at the upper right pole of the thyroid gland, and three thyroid tumors were found in the bilateral lobes of the thyroid gland. Histological diagnosis was adenoma of the parathyroid gland and follicular adenomas of the thyroid gland. Following removal of the parathyroid tumor, the serum calcium level rapidly dropped and the symptoms rapidly improved. Hypercalcemic crisis has a high mortality, and the only treatment is surgical removal. Therefore, in patients with hypercalcemic crisis due to primary hyperparathyroidism, surgical removal should be done immediately, unless the serum calcium level has dropped and symptoms of crisis have disappeared after conservative treatment.