The aim of this study was to investigate the relationship between heparanase expression and prognostic factors in endometrial cancer, as well as the relationship between heparanase expression during phases of the normal endometrial cycle. Immunohistochemical analysis of 166 endometrial cancers and 34 normal endometria in various phases of growth was performed. The heparanase expression in the late-proliferative phase of normal endometria was found to be significantly higher than in either the early-proliferative or the secretory phases (P = .012 and P = .044, respectively). Heparanase expression was also significantly higher in endometrial cancer patients with tumors of an advanced FIGO stage (P = .0003) and high FIGO grade (P = .004) and with cancers showing either deep myometrial invasion (P = .023), lymph node metastasis (P = .006), lymphvascular space involvement (P = .048), or positive peritoneal cytology (P = .010). The disease-free and overall survival rates of patients with intense heparanase expression were significantly lower than those of patients with absent or moderate heparanase expression (P = .004 and P = .002, respectively). Heparanase may participate in normal endometrial remodeling and can serve as an indicator of the aggressive potential and poor prognosis of endometrial cancers.