The patient was admitted to the hospital with a diagnosis of acute gastritis and dehydration. She was afebrile, normotensive, mildly dehydrated, and in no distress. Her sexual maturation was Tanner stage IV. Pelvic examination was deferred initially, and the remaining physical examination results were normal. Results of initial laboratory tests were normal except for a total serum calcium level of 14.1 mg/dL, an ionized calcium level of 7.46 mg/dL (reference range, 4.60-5.30 mg/dL), and a serum phosphorus concentration of 2.2 mg/dL. The urine calcium/creatinine ratio was elevated at 0.53 (reference range, 0.05-0.25), and the result of a pregnancy test was negative. A technetium Tc-99m sestamibi scan showed no evidence of parathyroid adenoma or ectopic parathyroid. A thyroid function test, electrocardiogram, and an abdominal radiograph were all normal. The patient responded well to therapy for symptomatic control of hypercalcemia (ie, intravenous fluids, potassium phosphate, furosemide, calcitonin, and pamidronate). The total serum calcium level fell to 9.6 mg/dL.