We report two patients with transient late neonatal hypocalcemia. Serum examination showed low calcium (Ca), high phosphorus, high parathyroid hormone (PTH), normal 25-hydroxyvitamin D and low or normal 1,25-dihydroxyvitamin D (1,25(OH)2vitD) levels. PTH infusion test revealed normal cyclic AMP production. In these patients, 1,25(OH)2vitD production by PTH, or Ca mobilization from bone and Ca absorption from intestine by PTH and 1,25(OH)2vitD might have been disturbed transiently due to developmental immaturities. This is a new type of transient late neonatal hypocalcemia with high serum PTH levels.