The histologic features of 187 cases of cystosarcoma phyllodes of the breast were reviewed. The tumors were divided into histologically benign, borderline, and malignant categories. Correlation with clinical outcome was available in 100 cases. Overall rate of local recurrence was 28% (benign, 27%; borderline, 32%; malignant, 26%). Metastases occurred in eight of 100 cases (two borderline and six malignant). Although no histologic features were predictive of local recurrence, stromal overgrowth, mitotic rate greater than 15 per 50 high-power fields, and cytologically atypical stromal cells characterized seven of the eight tumors that metastasized. These features were not evident in the eighth case. Flow cytometric analysis of eight tumors (four benign, two borderline, and two malignant) showed discordance between histology and DNA content in three cases. There was slightly better correlation of histology and S-phase fractions. Based on these results demonstrating the difficulty in predicting clinical outcome, wide local excision remains an appropriate initial method of treatment. Simple mastectomy may be necessary for very large tumors and should be considered in histologically malignant tumors and cases with multiple recurrences, since some recurrent tumors in this series showed increasingly unfavorable histologic features.