Apelin is a recently discovered peptide ligand reported to be involved in the regulation of cardiovascular homeostasis. The exact role of apelin in the pathophysiology of congestive heart failure has remained obscure, and the reported circulating levels of apelin in patients with heart failure have been contradictory. To establish the role of apelin in the assessment of cardiac dysfunction we measured plasma apelin levels in 65 patients with congestive heart failure caused by idiopathic dilated cardiomyopathy (IDC) and 14 healthy volunteers by specific radioimmunoassay. IDC patients were carefully examined including echocardiography, both-sided cardiac catheterization and cardiopulmonary exercise test. In addition, plasma levels of N-terminal pro-brain natriuretic peptide (NT-proBNP), N-terminal pro-atrial natriuretic peptide (NT-proANP), interleukin (IL)-6, tumor necrosis factor alpha (TNF-alpha), epinephrine and norepinephrine were determined. Plasma apelin levels were similar in IDC patients (median 26.5 pg/ml, range<3.40-97.6 pg/ml) and in control subjects (median 24.1 pg/ml, range 19.0-28.7 pg/ml; p=NS). Unlike the levels of NT-proBNP, IL-6, TNF-alpha, and norepinephrine, plasma apelin levels did not reflect the severity of heart failure. Our study demonstrates that although disturbed apelin-APJ signalling in heart may play a role in the pathophysiology of heart failure, circulating apelin levels cannot be applied in the clinical assessment of patients with chronic left ventricular dysfunction.