INTRODUCTION: Disorder in the function of the ARID1A gene is associated with the initiation and progression of many malignant tumors. In gastric cancer, ARID1A loss is noted in approximately 31% of all adenocarcinomas, in particular, in the subtype with microsatellite instability, indicating a probable potential for assessment of ARID1A protein for early diagnosis of early and pronounced precancerous alterations and gastric cancer. AIM: To compare the characteristics of ARID1A protein production in tissues of gastric adenocarcinoma and gastric mucosa in the distant zone of tumor growth to assess diagnostic and predictive potential of this marker. MATERIALS AND METHODS: Fragments of gastric mucosa of 43 patients with gastric adenocarcinoma were studied: group 1 — fragments of adenocarcinoma tissue (n = 43), group 2 — fragments of mucosa at 1 cm and more distance from tumor (distant zone, n = 43). The samples were subjected to immunohistochemical examination using ARID1A (ab182560, 1:1000; Abcam, UK). The intensity of nuclei staining (0 — none, 1 — weak, 2 — moderate, 3 — pronounced) and spread of immunohistochemical marks (0 — < 50% of stained nuclei, 1 — 50%–75%, 2 — 75–90%, 3 — 90%–95%, 4 — 95%–98%, 5 — more than 98%). were assessed. The evidence and spread scores were interpreted as ARID1A index. The data were presented as median and interquartile range. Mann-Whitney U test and Wilcoxon matched-pairs test were used to compare groups. RESULTS: ARID1A index demonstrated lower values in adenocarcinoma (4 [2–6]) compared to the distant zone (5 [3–6], p = 0.0447). However, the interquartile range of both groups partially overlapped, due to which, when using Wilcoxon test, no statistically significant difference (р = 0.0697) was found between matched pairs. CONCLUSION: The absence of statistically significant differences between the tumor tissue and distant zone of tumor growth in the assessment of matched samples can indicate the early involvement of pathways leading to loss of AID1A in the gastric mucosa, in carcinogenesis, and makes it a possible marker of the risk of development and early diagnosis of gastric cancer.