Background: Prognostic factors for diffuse large B-cell lymphoma (DLBCL) at day 100 post-autologous peripheral blood hematopoietic stem cell transplantation (APBHSCT) have not been evaluated. We previously reported that absolute monocyte (AMC)/lymphocyte count (ALC) prognostic score (AMLCPS) at diagnosis of DLBCL patients is a prognostic factor of survival, stratifying patients into three risk groups: low- (AMC<630/μL and ALC>1000/μL), intermediate- (AMC ≥ 630/μL or ALC ≤ 1000/μL) and high-risk (AMC ≥ 630/μL and ALC ≤ 1000/μL) (Leukemia 25: 1502-9, 2011). Therefore, we evaluated day 100 AMLCPS as a prognostic factor of survival by landmark analysis from day 100 post-APBHSCT in DLBCL patients. Methods: DLBCL patients that achieved complete response by day 100 post-APBHSCT qualified for the study. From 2000 to 2007, 134 consecutive DLBCL patients were evaluated. Overall survival (OS) and progression free survival (PFS) were calculated using Kaplan-Meier analysis. Results: The median follow up from day 100 for the cohort was 5.5 years (range: 0.17-12.17 years) and for living patients (N=93) was 6.9 years (range: 2.5-12.17 years). Day 100 AMLCPS was able to stratify patients into three risk groups low-, intermediate-, and high-risk for OS [low: median OS = not reached, 5-year survival rate = 94% (95%CI 83.3%-98.1%); intermediate: median OS = not reached, 5-year survival rate = 70% (95%CI 58.0%-79.8%); high: median OS = 2.2 years, 5-year survival rate = 13% (95%CI 3.4%-40.5%); p<0.0001] and PFS [low: median PFS = not reached, 5-year progression free rate = 87% (95%CI 74.5%-94.3%); intermediate: median PFS = 10.9 years, 5-year progression free rate = 67% (95%CI 55.4%-77.5%); high: median PFS = 1 year, 5-year progression free rate = 13% (95%CI 3.4%-40.5%); p<0.0001]. The day 100 AMLCPS was balanced in relation to the International Prognostic Index (IPI; p=0.22), infused CD34+ stem cells (p=0.92), and disease status pre-APBHSCT (complete remission versus partial response; p=0.11). Conclusions: The day 100 AMLCPS is a simple biomarker that can help identify DLBCL patients post-APBHSCT with poor clinical outcomes.