We investigated the efficacy of a Wilms' tumor gene 1 (WT1) vaccine combined with gemcitabine (GEMWT1) and compared it with gemcitabine (GEM) monotherapy for advanced pancreatic ductal adenocarcinoma (PDAC) in a randomized phase II study. We randomly assigned HLA-A*02:01- or HLA-A*24:02-positive patients with advanced PDAC to receive GEMWT1 or GEM. We assessed WT1-specific immune responses via delayed-type hypersensitivity (DTH) to the WT1 peptide and a tetramer assay to detect WT1-specific cytotoxic T lymphocytes (WT1-CTL). Of 91 patients enrolled, 85 were evaluable (GEMWT1: n = 42; GEM: n = 43). GEMWT1 prolonged progression-free survival [PFS; hazard ratio (HR), 0.66; P = 0.084] and improved overall survival rate at 1 year (1-year OS%; GEMWT1: 35.7%; GEM: 20.9%). However, the difference in OS was not significant (HR: 0.82; P = 0.363). These effects were particularly evident in metastatic PDAC (PFS: HR 0.51, P = 0.0017; 1-year OS%: GEMWT1 27.3%; GEM 11.8%). The combination was well tolerated, with no unexpected serious adverse events. In patients with metastatic PDAC, PFS in the DTH-positive GEMWT1 group was significantly prolonged, with a better HR of 0.27 compared with the GEM group, whereas PFS in the DTH-negative GEMWT1 group was similar to that in the GEM group (HR 0.86; P = 0.001). DTH positivity was associated with an increase in WT1-CTLs induced by the WT1 vaccine. GEM plus the WT1 vaccine prolonged PFS and may improve 1-year OS% in advanced PDAC. These clinical effects were associated with the induction of WT1-specific immune responses. Cancer Immunol Res; 6(3); 320-31. ©2018 AACR.