Fibroadenoma is the most frequent breast tumor in women aged 10 to 20 years, a neoplasm characterized by a benign multiplication with rapid growth and usually in the form of a single lump. A clinical case report study was performed on a 12 year and 7 month old girl, who had a left breast tumor, occupying all its extension. When the tumor is larger than 5 cm or 500 g or when it occupies at least 80% of the breast is considered a giant juvenile fibroadenoma. The choice of tumor surgical excision is the best option for both diagnosis and treatment, due to the need for pathologic evaluation. It is a picture that needs attention, since it has the characteristic of mimicking other pathologies, besides having a very rapid evolution.