We present two unrelated cases of young adults with hypercalcemia, hypercalciuria, and nephrocalcinosis. Both had suppressed intact parathyroid hormone levels and high 1,25 vitamin D levels after only brief, low-dose, over-the-counter vitamin supplementation. Neither had evidence of a granulomatous disorder. Their presentation mimicked that of 1,25 hydroxy vitamin D intoxication. In both patients, the diagnosis of idiopathic infantile hypercalcemia was confirmed with immeasurably low 24,25 vitamin D levels. Both were found to have a loss-of-function mutation in the CYP24A1 gene, which encodes the vitamin D-metabolizing enzyme 25-hydroxyvitamin D 24-hydroxylase.