Background: The optimal serum bicarbonate level is controversial for patients who are undergoing hemodialysis (HD). In this study, we analyzed the impact of serum bicarbonate levels on mortality among HD patients. Methods: Prevalent HD patients were selected from the Clinical Research Center registry for End Stage Renal Disease cohort in Korea. Patients were categorized into quartiles according to their total carbon dioxide (tCO2) levels: ‘quartile 1’, a tCO2 of <19.4 mEq/L; ‘quartile 2’, a tCO2 of 19.4-21.5 mEq/L; ‘quartile 3’, a tCO2 of 21.6-23.9 mEq/L; and ‘quartile 4’, a tCO2 of ≥ 24 mEq/L. Cox regression analysis was used to calculate the adjusted hazard ratio (HR) and confidence interval (CI) for mortality. Results: We included 1,159 prevalent HD patients, with a median follow-up period of 37 months. Kaplan-Meier analysis revealed that the all-cause mortality was significantly higher in patients from quartile 4, compared to those from quartile 3 (the reference category) (p=0.009, log-rank test). The multivariate Cox proportional hazard model revealed that patients from quartile 4 had significantly higher risk of mortality than those from quartile 3,after adjusting for the clinical variables in model 1 (HR: 1.99, 95% CI: 1.15-3.45, p=0.01) and model 2 (HR: 1.82, 95% CI: 1.03-3.22, p=0.04). Conclusions: Our data indicate that high serum bicarbonate levels (≥24 mEq/L) are associated with increased mortality in chronic HD patients. These findings suggest that efforts are needed to decrease the dialysate bicarbonate concentration or correct metabolic alkalosis in chronic HD patients.