PURPOSE: Malignant phyllodes tumors are rare lesions of the breast. The purpose of this study was to evaluate the clinical charateristics and the recurrence patterns of malignant phyllodes tumors. METHODS: Twelve patients with malignant phyllodes tumors who had undergone surgery at the Breast Clinic, Asan Medical Center, from February 1991 to October 1997 were analyzed retrospectively. Histologic criteria of malignant phyllodes tumor was more than 10 mitoses/10 high-power fields in this study. RESULTS: The mean age of the patients was 40.7 years. As regards to preoperative histological confirmation, malignant phyllodes tumors were proven in all 6 cases with an excision biopsy, but in only one of the 4 cases with fine needle aspiration. All the patients received surgical treatment: a modified radical mas tectomy in 4 cases, a simple mastectomy in 5 cases, a simple mastectomy with reconstruction in 2 cases, and a subcutaneous mastectomy in one case. During 34 months of mean follow-up, 3 patients had a recurrence. One patient had a chest wall recurrence at 29 months and lung metastasis at 48 months; the others had lung metastasis at 14 and 16 months, respectively. CONCLUSION: Malignant phyllodes tumors were difficult to confirm with fine needle aspiration preoperatively. The lung was the main site of metastasis, and if metastasis occurred, the prognosis was very poor.