The Fas/Fas Ligand system is a major apoptosis signaling pathway that is up-regulated in patients with chronic heart failure (CHF). Serum soluble Fas (sFas) levels increase in proportion to the CHF severity and may have prognostic value, therefore, sFas is a promising biomarker of heart failure. In this study, we attempted to identify the determinants of sFas levels in patients with CHF. Serum levels of tumor necrosis factor (TNF)-α and its soluble receptors (sTNF-R1 & sTNF-R2), interleukin (IL)-6, soluble IL-6 receptor (sIL-6R), glycoprotein (gp)130, and sFas were measured in 106 patients with CHF and 39 controls. All subjects performed a symptom-limited cycle ergometer exercise test with expired gas analysis. CHF patients had higher levels of TNF-α, sTNF-R1, sTNF-R2, IL-6, and gp130. Serum levels of sFas (controls versus CHF; 2.60 ± 0.88 versus 3.38 ± 1.23 ng/mL, P = 0.0004) were higher in CHF. On univariate analysis, age (P = 0.0003), NYHA functional class (P = 0.0012), peak VO₂ (P < 0.0001), plasma norepinephrine (P = 0.0013), log IL-6 (P < 0.0001), log TNF-α (P = 0.0002), log sTNF-R1 (P < 0.0001), and log TNF-R2 (P < 0.0001) were significantly related to log sFas levels. Multivariate analysis showed that age and log IL-6 and log sTNF-R1 levels were independently associated with log sFas levels (overall R = 0.603, P < 0.0001). Serum levels of sFas were increased in patients with CHF, and age and serum IL-6 and sTNF-R1 levels were independent determinants of sFas levels. These data suggest that proinflammatory cytokine activation is linked to the Fas/Fas Ligand system in patients with CHF.