The authors report a case of cystosarcoma phyllodes occurring in an adolescent female, thus raising to 19 the number of reported cases. The prognosis of those composite fibro-epithelial tumours is ruled by the malignancy of the connective component: the diagnosis shows no distinctive feature compared with what can be observed in adults. The prognosis (classically better) appears in fact totally similar, justifying the same therapeutic rigour, particularly in the surgical field. A multidisciplinary approach should ensure complete recovery while limiting the harmful effects of the therapy in very young women.