Objective - the study of the diagnostic significance of tumor markers СА125 and СА15.3 in patients with ovarian and breast cancer. Material and methods. 113 women were subdivided into the groups: 16 healthy donors and 16 patients with benign ovarian and breast tumors; 22 patients with various malignancies, excluding breast cancer and ovarian cancer; 30 patients with synchronous and 29 with metachronous multiple primary ovarian and breast cancer. Retrospective analyses of CA125 and CA15.3 levels were performed both in the control groups and in patients with multiple primary malignant neoplasms (MPMN) before the treatment. Tumor marker levels were determined by electrochemiluminescence method on the automatic analyzer ELECSYS 2010 and diagnostic test systems produced by Hoffmann-La Roche. Results. Diagnostic specificity of CA125 with discriminative level of 35 U/ml is very low while the discriminative level of more than 200 U/ml allows to achieve the 98% specificity and 100% sensitivity. Diagnostic specificity of СА15.3 was 94%, and sensitivity - 33%. Conclusion. CA125 has large diagnostic potential in detecting of ovarian cancer in patients with both synchronous and metachronous tumors, if discriminative level is more than 200 U/ml in multiple primary ovarian and breast cancer patients. Sensitivity of СА15.3 related to metachronous polyneoplasia is low.