Endometrial cancer is the most common malignant neoplasm of the female reproductive system. The number of diagnosed cases is increasing every year. In recent years, the triple-negative phenomenon (TNP) has been identified as one of the determinants of shorter survival in endometrial cancer patients. The aim of the study was to compare the PARP-1 protein expression in triple-negative (TNEC) and non-triple-negative (NTNEC) endometrial cancer patients and determine the relationship between the PARP-1 protein expression in endometrial cancer cells and patient's overall survival depending on the adopted scale (H-SCORE < 75, H-SCORE < 50, Allred scale). The study involved 265 patients with histopathologically confirmed endometrial cancer. The patients were divided into 2 groups: patients with TNEC and patients with NTNEC. The study was conducted using a tissue microarray technique. Expression of PARP-1 protein was determined by immunohistochemistry. Protein expression evaluation was performed using virtual microscopy and the Image Scope computer image analysis system. The following conclusions were reached: total and individual levels of nuclear or cytoplasmic PARP-1 expression varied with the presence or absence of TNP, and PARP-1 nuclear expression at the 2+ level had a significant effect on the increased risk of death (according to H-SCORE < 75).