Background and Aims. Fibronectin containing the extra domain A (ED-A(+) Fn) was proven to serve as a valuable biomarker for cardiac remodeling. The study was aimed at establishing an ELISA to determine ED-A(+) Fn in serum of heart failure patients. Methods. ED-A(+) Fn was quantified in serum samples from 114 heart failure patients due to ischemic (ICM, n = 44) and dilated (DCM, n = 39) cardiomyopathy as well as hypertensive heart disease (HHD, n = 31) compared to healthy controls (n = 12). Results. In comparison to healthy volunteers, heart failure patients showed significantly increased levels of ED-A(+) Fn (p < 0.001). In particular in ICM patients there were significant associations between ED-A(+) Fn serum levels and clinical parameters, for example, increased levels with rising NYHA class (p = 0.013), a negative correlation with left ventricular ejection fraction (p = 0.026, r: -0.353), a positive correlation with left atrial diameter (p = 0.008, r: 0.431), and a strong positive correlation with systolic pulmonary artery pressure (p = 0.002, r: 0.485). In multivariate analysis, ED-A(+) Fn was identified as an independent predictor of an ischemic heart failure etiology. Conclusions. The current study could clearly show that ED-A(+) Fn is a promising biomarker in cardiovascular diseases, especially in heart failure patients due to an ICM. We presented a valid ELISA method, which could be applied for further studies investigating the value of ED-A(+) Fn.