Phyllodes tumor of the breast is an unusual tumor with an incidence of I in 100,000. In particular, it is a very rare neoplasm in adolescent girls and young women. The authors present a case of a 13-year-old adolescent girl with a large unilateral palpable mass in her right breast. The diagnosis of cystosarcoma phyllodes was made in a frozen section after wide local excision. The manage­ment and the cytological and histological characteristics are described with particular reference to the very unusual clinical presen­tation in this patient.