BACKGROUND: Many studies have assessed the volume-outcome relationship in cancer patients, but most focused on better outcomes in higher volume groups rather than identifying a specific threshold that could assist in clinical decision-making for achieving the best outcomes. The current study suggests an optimal volume for achieving good outcome, as an extension of previous studies on the volume-outcome relationship in stomach cancer patients. METHODS: We used National Health Insurance Service (NHIS) Sampling Cohort data during 2004-2013, comprising healthcare claims for 2550 patients with newly diagnosed stomach cancer. We conducted survival analyses adopting the Cox proportional hazard model to investigate the association of three threshold values for surgical volume of stomach cancer patients for cancer-specific mortality using the Youden index. RESULTS: Overall, 17.10% of patients died due to cancer during the study period. The risk of mortality among patients who received surgical treatment gradually decreased with increasing surgical volume at the hospital, while the risk of mortality increased again in "high" surgical volume hospitals, resulting in a j-shaped curve (mid-low = hazard ratio (HR) 0.773, 95% confidence interval (CI) 0.608-0.983; mid-high = HR 0.541, 95% CI 0.372-0.788; high = HR 0.659, 95% CI 0.473-0.917; ref = low). These associations were especially significant in regions with unsubstantial surgical volumes and less severe cases. CONCLUSION: The optimal surgical volume threshold was about 727.3 surgical cases for stomach cancer per hospital over the 1-year study period in South Korea. However, such positive effects decreased after exceeding a certain volume of surgeries.