Cystosarcoma phyllodes (CP) is an uncommon tumor in adolescent girls and young women. This study seeks to define the clinical and pathologic features of CP in this unusual clinical setting. Forty-five CPs (34 benign and 11 malignant) in prepubertal and adolescent girls and young women were studied. Classification of the tumors was based on the following morphologic features: stromal cellularity, nuclear atypia, mitotic activity, necrosis, and the nature of tumor borders. Surgical therapy was local excision or mastectomy. The age of the patients ranged from 10 to 24 years (mean 17.7). The tumors measured 1.4 cm to 10.2 cm at their widest point (mean 4.6). Both breasts were affected equally. Thirty-two patients were treated by local excision with or without reexcision and four patients by mastectomy. Follow-up was available for 36 patients for a mean of 58.4 months. Local recurrence was reported in six of the 36 cases (16%) (four benign and two malignant). The six patients with recurrent disease had infiltrative tumor borders and positive surgical margins microscopically. There was a single instance of systemic metastases from a high-grade malignant tumor with rhabdomyosarcomatous stromal differentiation and a high mitotic rate. At last follow-up, 34 patients were alive with no evidence of disease, one patient was alive with pulmonary metastases, and one patient died of an unrelated cause. We concluded that CP in adolescent girls and young women is not more aggressive than in older patients. Infiltrative tumor borders and positive surgical margins are the best histologic predictors for local recurrence. Mitotic activity is the most important criterion for assessing the metastatic potential. CP in this age group should be treated to maximize breast conservation. Mastectomy may be required to obtain clear margins for CPs that cannot be managed by excision because of large tumor size relative to breast volume.