Juvenile gigantomastia in a 12-year-old girl was treated by a bilateral reduction mammoplasty with free transplantation of the areolae and nipples and the removal of 3,980 g of breast tissue. Regrowth of the residual breast tissue has been suppressed by the administration of tamoxifen, an antiestrogen drug, since the surgery. This case was positive for estrogen receptors (ER) by the dextran-coated charcoal method, while tissue staining for ER and estradiol resulted in a darker staining of the epithelial contents, especially of fibroadenoma-like nodules, using an immunocytochemical assay. It is thus suggested that the etiology of this disease might be related to a local hypersensitivity to estrogen.