Endometrial cancer is a common type of cancer in women, with endometrial adenocarcinoma (EA) being the most common type. Monitoring the expression levels of signal transducer and activator of transcription 3 (STAT3), the protein levels of interleukin 6 (IL-6) and soluble programmed death ligand 1 (sPD-L1), and their differences in patients with various pathological characteristics is beneficial for accurately evaluating the disease stage and differentiation degree of patients in clinical practice. The aim of the present study was to assess the expression levels of STAT3, and the protein levels of IL-6 and sPD-L1 in EA. In the present retrospective study, data were retrieved from the medical records of 137 patients with EA who received surgical treatment at The First Affiliated Hospital of Hebei North University from January 2017 to December 2022. Of the 137 cases, 90 met the inclusion criteria. The patients with EA were matched with a cohort of 30 patients with atypical endometrial hyperplasia in a ratio of 3:1. Among the 90 patients with EA, 30 patients with well-differentiated EA were matched with 30 patients with moderately differentiated EA and 30 patients with poorly differentiated EA in a 1:1:1 ratio. Expression level of STAT3, and protein levels of IL-6 and sPD-L1 were recorded preoperatively and compared between patients with different pathological characteristics [such as differentiation degree, disease stage, depth of myometrial invasion and lymph node metastasis (LNM)] and prognosis. Levels of IL-6, STAT3 and sPD-L1 in the observation group were significantly higher compared with the control group (P<0.001). Additionally, there were significant differences in IL-6, STAT3 and sPD-L1 levels between patients with different differentiation degrees, disease stages, myometrial invasion and LNM (P<0.001). The increase in IL-6, STAT3 and sPD-L1 levels were significantly associated with the decrease in the differentiation degree and the increase in the disease stage, depth of myometrial invasion and LNM (P<0.001). IL-6, STAT3 and sPD-L1 levels in patients with a poor prognosis were significantly higher compared with patients with good prognoses (P<0.001). Overall, the expression levels of STAT3, and the protein levels of IL-6 and sPD-L1 were increased in patients with EA compared with in those without EA, and their increase is associated with the pathological characteristics of the disease. The levels of these indices may be detected in clinical practices to evaluate the disease and predict the prognosis.