Authors performed an immunohistochemical analysis using monoclonal antibody to p53 protein on 15 cases of benign and malignant phyllodes tumor of the breast along with a review of other conventional clinicopathological parameters to investigate the meaning of p53 expression. The cases were composed of 8 benign and 7 malignant lesions. The pattern of p53 expression showed a statistically significant difference between these benign and malignant lesions (p < 0.005). None of the benign cases expressed p53 whereas 6 out of 7 malignant cases did. Among malignant phyllodes tumors, the pattern of expression was diffuse and strong in two cases while granular and relatively weak in the remaining 4 cases. p53 expression seemed to be a unique feature of malignant phyllodes tumors, thereby, one of the most significant parameters for the differentiation of benign and malignant phyllodes tumors of the breast.