PURPOSE: Malignant phyllodes tumors are rare breast tumors. Information on the prognosis and optimal treatment of these lesions is not yet sufficient. The aim of this study was to determine parameters that predict the recurrence of malignant phyllodes tumors of the breast. METHODS: Retrospectively, we reviewed the medical records and pathological slides of 23 patients with malignant phyllodes tumors that had undergone surgical treatment from 1988 to 2006. The age of the patients, tumor size, type of surgery, resection margin, adjuvant therapy and pathological characteristics of the tumors such as stromal hypercellularity, cellular phleomorphism, mitosis, margins, and stromal pattern were examined. RESULTS: The mean age of the patients was 41 yr. The tumor size ranged from 1 cm to 25 cm, with a median of 7.42 cm. The median follow-up time was 29.0 months. Recurrence was observed in 6 patients (26.1%) and the 5-yr disease free survival was 48.9%. Risk factors for recurrence of a malignant phyllodes tumor were a mitotic index greater than 10 per high-powered field (p=0.0242) and an invasive margin (p=0.0437). CONCLUSION: Frequent mitosis and an invasive margin were the principal determinants of recurrence. Patients with poor prognostic components should be treated more aggressively and the patients need more close follow-up.