Breast enlargement with bloody discharge is very rare in childhood. We report a two year-old boy with breast enlargement and bloody discharge. Because of persistent bloody discharge, subcutaneous mastectomy was performed. The specimen showed histologic changes identical to those seen in adult mammary duct ectasia. We suggested that the infant's own endocrine system is responsible for breast enlargement and mammary duct ectasia, possibly occurring as a result of a mechanism similar to that in adults.