Neoadjuvant therapy (NAT) for early-stage pancreatic ductal adenocarcinoma (PDA) has recently gained prominence. We investigated the clinical significance of mucin 5 AC (MUC5AC), which exists in two major glycoforms, a less-glycosylated immature isoform (IM) and a heavi-ly-glycosylated mature isoform (MM), as a biomarker in resected PDA. Immunohistochemistry was performed on 100 resected PDAs to evaluate the expression of IM and MM isoforms of MUC5AC using respective monoclonal antibodies CLH2 (NBP2-44455) and 45M1 (ab3649). MUC5AC localization (cytoplasmic, apical, and extra-cellular, EC) was determined, and H-scores were calculated. Univariate and multivariate (MVA) Cox regression models were used to estimate progression-free survival (PFS) and overall survival (OS). Of 100 resected PDA patients, 43 received NAT, and 57 were treatment-naïve with upfront surgery (UpS). In the study population (N=100), IM expression (objective response vs. no response vs. UpS = H-scores 104 vs. 152 vs. 163, p=0.01) and MM-MUC5AC detection rates (56% vs. 63% vs. 82%, p=0.02) were significantly different. In the NAT-group, MM-MUC5AC-negative patients had significantly better PFS on MVA (Hazard Ratio, 0.2, 95 % CI: 0.059-0.766, p = 0.01). Similar results were noted in a FOLFIRINOX sub-group (N=36). We established an association of MUC5AC expression with treatment response and outcomes.