The authors, in order to obtain a diagnostic index for phyllodes tumors and identified histological parameters that will predict the clinical course of this neoplasm, developed a histological degree of aggressiveness based on specific histological parameters, including: stromal:gland ratio, tumor margins, mitotic index and degree of stromal pleomorphism. Three categories were established: benign, intermediate and malignant. The probability of recurrence was estimated by the relative risk and by a multivariate Cox analysis. A strong and significant association was observed between this histological index and recurrence. The relative risk was 6.0 for intermediate lesions and 11.4 for malignant lesions when compared with the benign category. The microscopic examination of all axillary lymph nodes was negative for metastatic disease. In the multivariate analysis, the stroma:gland ratio was the strongest predictor for recurrence. These results indicate that by assigning a numerical value to certain histopathologic variables a better correlation with the clinical outcome of the patient can be obtained.