We evaluated the safety of, and clinical and immune responses to personalized peptide vaccination with gemcitabine (GEM) as the first line therapy in patients with non-resectable pancreatic cancer. Pre-vaccination peripheral blood mononuclear cells (PBMCs) and plasma were prepared to examine cellular and humoral responses to 14 and 16 peptides in human leukocyte antigen (HLA)-A24+ or -A2+ patients, respectively. Only the reactive peptides (maximum of 4) were administered weekly at 3 mg/peptide. GEM was administered at 1000 mg/m(2) per week for 3 weeks, followed by 1 week of rest. Twenty-one patients with untreated and non-resectable pancreatic cancer were enrolled. The combination therapy was generally well tolerated. Boosting of cellular and humoral responses to the vaccinated peptides was observed in the post-vaccination (eighth) PBMCs and plasma from 14 of 18 and 13 of 18 patients tested, respectively. The best clinical responses were 7 cases of partial response, 9 cases of stable disease, and 5 cases of progressive disease. Median survival time of all 21 patients was 9.0 months (95% CI, 6-15.5 months) with a one year survival rate of 38%. Immune boosting in both cellular and humoral responses was well correlated with overall survival with a hazard ratio of 0.2 (95% CI, 0.06-0.73; log-rank p=0.0239). These results suggest a potential clinical benefit of this combination therapy for non-resectable pancreatic cancer patients as the first line therapy. Further exploration of this approach is warranted.