The authors report a case of giant fibroadenoma of the breast in a girl of 11 years old. Juvenile or giant fibroadenoma is a rare pathology usually presenting in adolescence, characterized by massive and rapid enlargment of an encapsulated mass. Nowdays there are some preoperative difficulties distinguishing it from cystosarcoma phyllodes which has a benign and malignant form. It is important to differentiate the two pathologies before operation as they have a different therapeutic approach and different follow up. The etiology is believed to be an end-organ hypersensitivity to normal levels of gonadal hormones and the age of presentation is between 10 to 18 years old. Treatment is usually surgical and ranges from simple excision to subcutaneous mastectomy with reconstruction.