Peroxidase activity was determined in 98 patients with endometrial cancers. In 34 patients with endometrial cancers, the activity of the enzyme was also detected in the neighbouring unchanged (normal) tissues. It was shown that in tumor tissues the level of peroxidase activity was higher than in normal ones. The peroxidase activity increased both in tumor and normal endometrium when clinical and morphological characteristics of tumor were unfavourable, i.e. at later stages of the disease, deeper invasion to myometrium, lower levels of differentiation of tumors. The peroxidase activity was somewhat higher in post-menopausal women; however, no direct was found between the activity of this enzyme in malignant and normal tissues, on the one hand, and the menstrual status of patients with different clinical and morphological characteristics of endometrial cancer, on the other relationship. The level of peroxidase activity in normal and malignant tissues was more pronounced in advanced and poorly-differentiated tumors. It seems important to use peroxidase activity as a prognostic factor and as a possible marker of endometrial cancer hormone-dependence that needs further investigation.