Background: To investigate the perioperative peripheral blood levels of CD4+CD25+ regulatory T cells, programed cell death 1 (PD-1), and lymphocyte activation gene 3 (LAG-3) in patients with advanced Siewert type II adenocarcinoma of esophagogastric junction (AEG). Methods: Patients (n = 102) with advanced Siewert type II AEG underwent open total gastrectomy/proximal gastrectomy with a transhiatal resection of the distal esophagus and lymphadenectomy of the lower mediastinum and the abdominal D2 compartment. Flow cytometry was used to detect CD4+CD25+ T cells, PD-1 and LAG-3 expression on both CD4+ and CD8+ T cells in the peripheral blood of the Siewert type II AEG patients prior to surgery and on postoperative day (POD) 1, 3, 7, and 9. Results: The proportion of CD4+CD25+ T cells rapidly decreased on POD 1, then gradually increased and peaked at POD 7. The proportion of CD4+PD-1+ T cells significantly increased after surgery, reaching a maximum on POD 1, and remained significantly elevated on POD 3 compared to the preoperative day. The proportion of CD8+ PD-1+ and CD4+LAG-3+ T cells gradually increased after surgery and reached a peak at POD 7. The change in proportion of CD8+LAG-3+ T cells in the peripheral venous blood lymphocytes after surgery was not statistically significant. Conclusion: The change in the CD4+PD-1+ T lymphocyte ratio may likely reflect the cellular immunity status of the perioperative period.