Introduction: Gastric cancer is one of the most common oncological diseases in the world, occupying the 5th place of morbidity and the 3rd place in the structure of mortality from oncological diseases. For a long time, issues affecting the risk of developing cancer after bariatric surgery remain relevant. Our observation is devoted to the diagnosis and treatment of a patient with gastric cancer after gastric bypass. Description of the clinical case: A 62‑year‑old patient was operated on 13. 07. 2016 – laparoscopic Roux gastric bypass for morbid obesity, 11. 09. 2019 – laparoscopic installation of the Cardioplant plate on a small stomach due to recurrent weight gain. Since 2020 the patient had the phenomena of anastomositis, dysphagia and gastroesophageal reflux. Courses of conservative therapy, sessions of balloon dilation of gastroenteroanastomosis and anastomosis resection failed to show a significant effect. The patient underwent comprehensive examinations at each treatment, including abdominal MSCT, cancer markers and studies of biopsy material of the gastric mucosa and gastroenteroanastomosis. As a result of histological studies, no signs of cancer were found. After applying to the Avicenna Medical Center in 2022 a molecular genetic analysis was carried out, in which the mRNA panel most corresponded to a malignant neoplasm. 20. 12. 2022 extirpation of the stomach stump with resection of the esophagus was performed. The cancer diagnosis was confirmed by histological and immunohistochemical studies: low‑grade adenocarcinoma of the stomach with a cricoid component with germination into the esophagus and small intestine, with spread beyond the muscle layer. Conclusion: This clinical case highlights the complexity of oncological verification in patients after bariatric surgery. Prolonged dysphagia, anastomositis and recurrent GERD in such patients determine the need for a more detailed examination, including the latest achievements of molecular genetic analysis.