Background: To investigate the targeting of indeolamine 2,3 dioxygenase (IDO) enzyme by a synthetic peptide vaccine for patients with metastatic non small-cell lung cancer (NSCLC). Methods: We treated15 HLA-A2 positive patients with stage III-IV NSCLC and disease stabilization (SD) after standard chemotherapy. Patients were treated every second week (induction) for three months, and thereafter monthly until progression (maintenance) with imiquimod ointment and vaccine (100 µg IDO5 peptide sequence ALLEIASCL mixed with 900 µL montanide) administered subcutaneously. Primary end point was toxicity. Clinical benefit and immune monitoring were assessed. Results: Patient characteristics: mean age (63, range 51-71), sex (F=8,M=7), PS(0=9,1=6), histology (adenocarcinoma=93%), previously anti-neoplastic treatments (1^st line=100%, 2^nd =40% and 3^rd=27% of patients). No grade 3-4 CTCAE toxicity was observed. Median PFS was 5.2 months and median OS 2.1 years. Long-lasting disease stabilization (SD≥8.5 months) was demonstrated in 7 patients (47%). Patients demonstrated significant improved OS (P=0.02) when compared to the untreated group of excluded HLA-A2 negative NSCLC patients. IDO expression was frequently detected in tumour biopsies by immune-histochemistry staining. Immune induction of IDO specific CD8 T-cells were demonstrated by IFN-y Elispot and Tetramer staining. Immune correlates of T-lymphocyte subsets were performed by flow cytometry. HPLC analyses of Trp/Kyn ratio suggested stabilization or decrease of IDO activity in 11/15 (73%) of the patients. Conclusions: The vaccine was safe and well-tolerated with no grade 3/4 toxicity occurring. Long-lasting SD was seen in 47% of the patients demonstrating a median OS of 2.1 year. IHC demonstrated frequent IDO activity in NSCLC tumour biopsies, and blocking of IDO activity was indicated by Kyn/Trp ratio measurements. Clinical trial information: NCT01219348.