Estrogen receptor (ER) is a major feature of endometrioid adenocarcinoma. It has a significant impact on constitution of estrogen-responsiveness of this endometrial malignancy, in which STAT3 (signal transducer and activator of transcription) becomes hyperactivated. The aim of our study was to detect immunohistochemically and compare expressions of STAT3 with apoptosis regulators (Bak and Bcl-xL) in regard to different pathological features and variably pronounced ER-α immunoprofile in 78 endometrioid adenocarcinomas. STAT3 was abundantly detected in nuclei of cancer cells in 54 cases, thus pointing at its activation as an universal nuclear transcriptional factor. Bcl-xL and Bak were expressed in cytoplasm of malignant cells in 62 and 20 cancers, respectively. STAT3 correlated both with Bcl-xL (p = 0.001, r = 0.365) and Bak (p < 0.001, r = 0.436) in all of endometrioid adenocarcinomas and variably in different subgroups of these tumours segregated in regard to grading, staging and patients' age. Remarkably, only ER-α positive cancers retained these correlations in opposition to ER-α negative tumours with negativity defined as an immunoreactivity below 10%. ER-α receptor probably enhances interactions between STAT3 and Bcl-xL to be present in statistically significant manner. Presence of ER-α receptor seems to be crucial for relationships among Bcl-xL and STAT3 to occur in endometrioid adenocarcinomas.