Aim: Granulysin (cytolytic molecules of cytotoxic T lymphocytes and natural killer cells) is synthesized as cytosolic 9-kDa and secretary 15-kDa isoforms. We evaluated the prognostic significance of the pretreatment serum level of 15-kDa granulysin in patients with diffuse large B cell lymphoma (DLBCL). Patients and Methods: A retrospective analysis was conducted on 88 DLBCL patients treated homogeneously with standard chemotherapy. The granulysin level was quantified in pretreatment samples. Results: The granulysin level in DLBCL patients was significantly lower than that in healthy controls (522 ± 496 vs. 1,945 ± 1,696 pg/ml; p < 0.0001), and the level in patients who experienced recurrence within 3 years was significantly lower than that of patients without recurrence (305 ± 337 vs. 720 ± 607 pg/ml; p = 0.001). Patients with granulysin levels higher than the median level showed significantly longer progression-free and overall survival according to univariate analysis (p = 0.031 and p = 0.014, respectively). In multivariate analysis, the granulysin level was an independently significant prognostic factor of overall survival (p = 0.018; hazard ratio, 0.521; 95% confidence interval, 0.188–0.841). Conclusions: Pretreatment serum level of 15-kDa granulysin may be a valuable prognostic marker in DLBCL patients treated with standard chemotherapy.