PURPOSE: Phyllodes tumor is a rare fibroepithelial tumor of the breast, first described by Johannes Muller in 1838. Much has been written about phyllodes tumor, but very few widely accepted conclusions about its clinical behavior, treatment modality, and prognosis have been reached. This study aims to analyze the clinical, radiological, and pathological characteristics of phyllodes tumor of the breast. METHODS: The medical records of 41 patients with phyllodes tumor who had been treated between February 1982 and August 1998 at the Department of Surgery, Seoul National University Hospital, were retrospectively reviewed for clinical, radiological and pathological findings, treatment modalities, and follow-up results. RESULTS: Of these 41 cases, there were 28 cases (68.3%) of benign tumors and 13 cases (31.7%) of malignanat tumors. All patients were females, and the mean ages of onset were 33.2 years for benign tumors and 40.8 years for malignant tumors. Most patients, 100% of benign and 92.3% of malignant, presented with a papable mass in the breast. The median duration of illness was 2 months for malignant tumors and 8 months for benign tumors. The tumor size was greater than 10 cm in diameter in 5 cases (38.5%) of malignant tumors and in 3 cases (10.7%) of benign tumors. Only 4 cases were preoperatively diagnosed as having a phyllodes tumor by using radiological and fine needle aspiration cytology. Out of the 10 malignant cases reviewed, 5 cases were confirmed as malignant, and 5 cases were confirmed as borderline phyllodes tumors. Cellularity was moderate or above in all 5 malignant and 5 borderline cases. Atypism above moderate degree was found in 4 of 5 (80%) malignant tumors, in 3 of 5 (60%) borderline tumors, and in 4 of 24 (16.7%) benign tumors. Mitotic counts in all 5 malignant cases were 5 or more per 10 high power field while those in the 5 borderline tumors were 2-5 mitoses per 10 high power field. The most commonly performed operative procedures were a simple mastectomy (50%) for malignant tumors and a simple excision (64.3%) for benign tumors. Post-operative adjuvant therapy was done for 4 cases; out of these, 1 case had been initially diagnosed as a malignant phyllodes tumor, but the diagnosis was changed to a benign phyllodes tumor upon review. Of the 27 follow-up cases, recurrences developed in 3 cases (16.7%) of benign tumors. CONCLUSION: From the above results, there were no specific clinical features for differentiating benign from malignant phyllodes tumor preoperatively; therefore, we cannot help depending on the pathologic findings. Pathologic reviews showed that among several criteria, atypism, cellularity, and mitotic count were the most definite pathologic characteristics in differentiating benign from malignant phyllodes tumor. But much more experience and long-term follow-up may be needed to define optimal treatments and to analyze the prognosis for phyllodes tumors of the breast.