Two cases of the breast cyst in children were reported. Case 1: three-year-old female having a breast mass during two years was transferred to the authors' hospital. Removal of the cystic mass was performed. Case 2: three-year-old male was admitted to the authors' hospital because of a rapid growth of a breast mass being noted from his birth. Resection of the cystic mass was done. Pathologically, both of the masses were diagnosed as retention cysts of the breast ductal system.
