Objective — to determine the nature of origin (multiple primary or metastatic) of synchronous endometrioid malignant tumors of the uterine corpus and the ovary, as well as to clarify the role of endometriosis in carcinogenesis of endometrioid ovarian tumors. Material and methods. Clinical morphological characteristics of patients with synchronous endometrioid multiple primary malignant tumors of the uterine corpus and the ovary (n=66) and of patients with endometrioid adenocarcinoma of the uterine corpus and the ovary (n=50) were analyzed. Molecular genetic technologies were used in the study aimed to clarify their clonal origin as well as to explore the role of endometriosis in carcinogenesis of endometrioid cancer. Results. Despite the significant differences in the clinical course of the disease as well in morphological features of the tumors in these two groups of patients, the results of the molecular genetic research undertaken indicated that endometrioid tumors had a single clonal origin and conformed to endometrial cancer with metastases to the ovaries in 100% of observations. When analyzing endometriosis foci the decrease of ARID1A expression was identified in 8 (80%) of 10 observations. Conclusion. Based on molecular genetic study it was revealed that endometrioid cancer of the uterine corpus and the ovary are presumably one single pathology and their induction and development are associated with endometriosis.