PurposeStrategies for Indolamine-2,3-dioxygenase 1 (IDO1) inhibition in cancer immunotherapy once produced encouraging results, but failed in clinical trials. Recent evidence indicates that immune cells in the tumour microenvironment, especially macrophages, contribute to immune dysregulation and therefore might play a critical role in drug resistance.MethodsIn this study, we investigated the significance of IDO1 expressing immune cells in primary tumours and corresponding lymph node metastases (LNMs) in oral squamous cell carcinoma (OSCC) by immunohistochemistry. The link between IDO1 and macrophages was investigated by flow cytometry in tumour tissue, healthy adjacent tissue and peripheral blood mononuclear cells (PBMCs). IDO1 activity (measured as Kynurenine/Tryptophan ratio) was assessed by ELISAs.ResultsHigh IDO1 expression in tumour-infiltrating immune cells was significantly correlated with advanced stages [Spearman’s rank correlation (SRC),p= 0.027] and reduced progression-free survival (multivariate Cox regression,p= 0.034). IDO1 was significantly higher expressed in PBMCs of patients in advanced stages than in healthy controls (ANOVA,p< 0.05) and IDO1+macrophages were more abundant in intratumoural areas than peritumoural (ttest,p< 0.001). IDO1 expression in PBMCs was significantly correlated with IDO1 activity in serum (SRC,p< 0.05). IDO1 activity was significantly higher in patients with LNMs (ttest,p< 0.01).ConclusionAll in all, IDO1 expressing immune cells, especially macrophages, are more abundant in advanced stages of OSCC and are associated with reduced progression-free survival. Further investigations are needed to explore their role in local and systemic immune response. The IDO1 activity might be a suitable biomarker of metastasis in OSCC patients.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00432-022-04277-7.