The diagnosis of primary hyperparathyroidism in children is often delayed and is usually based on symptoms of hypercalcemia rather than abnormal laboratory values alone. We report the case of an 8-year-old boy with hypercalcemia, hypophosphatemia and mildly, but inadequately elevated intact parathyroid hormone (iPTH) who presented without any symptoms of hyperparathyroidism. Although imaging studies were misleading and four normal parathyroid glands were found intraoperatively, exploration of the thymus revealed an ectopic parathyroid adenoma. After removal of the ectopic gland, a rapid iPTH immunoassay proved immediate normalization of iPTH. This is the first report of sporadic isolated primary hyperparathyroidism diagnosed in an asymptomatic child on the basis of hypercalcemia and hypophosphatemia.