Background/Aims: Baseline serum lactate dehydrogenase (LDH) level is a well-known prognostic factor in patients with non-Hodgkin's lymphoma; however, its role beyond initial diagnosis has not yet been defined. Methods: This study was conducted as a retrospective analysis of patients with diffuse large B cell lymphoma (DLBCL) treated with R-CHOP21, who had undergone regular checks for LDH during immunochemotherapy (n = 119) and during the posttreatment follow-up period after complete remission (CR; n = 100). The 119 patients were classified into 4 groups according to their baseline and change in LDH level during treatment, and an analysis of tumor response and survival was performed. The value of LDH as a predictor for relapse was evaluated among the patients with regular follow-up visits after achieving CR. Results: An increased LDH level during immunochemotherapy had no impact on tumor response or survival, and only the LDH status ‘before' treatment was a prognostic marker. The sensitivity, specificity, positive predictive value and negative predictive value of serum LDH for detecting relapse after CR were 47.4, 86.5, 9.3 and 98.3%, respectively. Conclusion: The measurement of LDH level beyond initial diagnosis has no clear benefit in predicting disease progression or relapse in patients with DLBCL treated with R-CHOP21.