Chronic heart failure (HF) is a state of inflammatory immune activation characterized by elevated circulating levels of tumor necrosis factor-α (TNF-α). Interleukin-10 (IL-10) is a potent anti-inflammatory cytokine that inhibits TNF-α production and lessens endotoxin bioactivity. It is not known whether IL-10 reduces lipopolysaccharide (LPS) stimulated TNF-α production of peripheral blood mononuclear cells (PBMCs) from patients with chronic HF. PBMCs were isolated from 15 patients with chronic HF (New York Heart Association functional class 3.0 ± 0.2, left ventricular ejection fraction 30 ± 2%, peak oxygen consumption 18.1 ± 0.8 ml/kg/min) and 15 healthy control subjects and stimulated with 1 and 10 ng/ml LPS for 24 hours with or without prior addition of IL-10 (10 ng/ml). TNF-α was quantified in cell-free supernatants by an enzyme-linked immunosorbent assay. TNF-α, soluble TNF receptors, IL-10, and LPS were quantified in plasma. LPS stimulated TNF-α production was highest in those patients in New York Heart Association class II (p <0.01 vs New York Heart Association class III and IV, p <0.001 vs control subjects). IL-10 reduced PBMC TNF-α production in all stimulated samples at 1 and 10 ng/ml LPS (mean reduction 43% at 1 ng/ml, p <0.01 and 55% at 10 ng/ml, p <0.0001). The percentage reduction in TNF-α release did not differ significantly between patients and control subjects or with respect to severity of chronic HF or baseline immune parameters. Independently of clinical severity, IL-10 profoundly inhibits TNF-α release from PBMCs isolated from patients with chronic HF. IL-10 is, therefore, a potential therapy for use in chronic HF associated with inflammatory immune activation.