Gigantomastia is a rare condition characterized by excessive breast growth. It has been reported that the majority of gigantomastia cases occur during either pregnancy or puberty. We were presented with a rare case of gigantomastia associated with neither pregnancy nor puberty, and successfully treated it with reduction mammaplasty and free nipple graft. This idiopathic gigantomastia is the very first case in Korea, and adds to the worldwide total of 9 reported cases.