In this paper we report a case of cystosarcoma phyllodes malignum in a 45 years old female patient. A local relapse occurred 20 months after ablatio mammae. This tumor was histologically a benign fibroadenoma. 69 months postoperative metastases of the pleural diaphragma were found: the appearance was similar to that of a mesothelioma. The CEA was helpful in revealing the metastases and in indicating the progression. This case is discussed and compared to the review of the literature. Because of the high rate of local relapses a postoperative irradiation of the chest wall is proposed.