Chronic heart failure is a state of immune activation, and endotoxin is a potential trigger for cytokine production. Our aim was to study whether immune activation and endotoxemia occur in adults with congenital heart disease. We prospectively measured tumor necrosis factor (TNF)-alpha, soluble TNF receptors (sTNFR-1, sTNFR-2), interleukin-6, interleukin-10, endotoxin, and soluble CD14 levels in 52 consecutive adults with congenital heart disease (age 34 +/- 2 years [mean +/- SEM]) and 18 healthy controls (age 31 +/- 1 years). A variety of congenital heart lesions were studied: single ventricle physiology (n = 15), systemic right ventricle (n = 7), tetralogy of Fallot (n = 20), and "other" congenital heart disease (n = 10). Patients were subgrouped into asymptomatic (New York Heart Association [NYHA] class I, n = 11), mild (NYHA class II, n = 30), and moderate/severe (NYHA class III/IV, n = 11) categories. Patients had elevated TNF and interleukin-6 levels compared with controls (TNF 2.8 vs 2.1 pg/ml, p <0.05; interleukin-6 8.5 vs 5.7 pg/ml, p <0.001). TNF levels were higher in patients with moderate/severe symptoms compared with patients who were asymptomatic or had mild symptoms (p <0.05). Soluble TNFR-1 levels related directly to the degree of systemic ventricular impairment (p <0.05). There were no significant differences in sTNFR-1, sTNFR-2, interleukin-10, or sCD14 levels between patients and controls. Endotoxin levels were greater in patients with congenital heart disease versus controls (0.40 vs 0.26 endotoxin units/ml, p <0.0001). Thus, adults with congenital heart disease have elevated levels of inflammatory cytokines and bacterial endotoxin, which relate to functional status. Congenital heart disease in adults may be amenable to novel anti-inflammatory therapies in selected patients.