Abstract Background Heart failure (HF) is a leading cause of mortality in adults with congenital heart disease (ACHD). ACHD is characterized by predominant right heart disease. In non-congenital heart disease inflammation and its mediators such as monocytes/macrophages play an important pathophysiological role in HF. We aimed to evaluate the role of circulating monocyte subsets in ACHD-HF. Methods This cross-sectional study includes 209 ACHD outpatients (mean age: 35.3±11.0 years; NYHA class I/II/III-IV 58.3%/19.6%/13%; male: 59.8%) and 21 healthy controls (age: 29.8±12.6 years; male: 47.6%). Patients with clinical signs of infection, inflammatory diseases or malignancies were excluded. Multivariate analysis was used to relate blood monocyte subsets to NYHA class and echocardiographically derived parameters of right and left ventricular function. Results Compared to control, ACHD had significantly higher circulating levels of pro-inflammatory HLA-DR+CD14++CD16+ intermediate monocytes (24.0±3.3 vs. 43.6±1.7 cells/μL; p<0.001). NT-proBNP was independently associated with reduced left (p<0.0001) and right (p<0.001) ventricular function, diastolic dysfunction (p=0.04) and vena cava diameter (p=0.02). Independent predictors of NYHA class were intermediate monocytes (p=0.022), plasma noradrenaline (p=0.002), albumin (p=0.001) and NT-proBNP (p<0.001). Elevated right ventricular systolic pressure (>35 mmHg) was independently associated with both, higher intermediate monocyte counts (OR 1.36; 95% CI: 1.13–1.62; p=0.001) and low oxygen saturation (OR 0.8; 95% CI: 0.7–0.92; p=0.001), even after multivariable adjustment for age, sex and NYHA class. Conclusions Right ventricular pressure and oxygen saturation are linked to elevated intermediate monocytes, suggesting an important link between inflammation and HF in ACHD. Circulating blood intermediate monocytes represent a promising biomarker in ACHD. Acknowledgement/Funding German Heart Foundation (Deutsche Herzstiftung e.V.)