Background-The etiology of ventricular dysfunction in adult congenital heart disease (ACHD) is not well understood.Diffuse fibrosis is a likely common final pathway and is quantifiable using MRI. Methods and Results-Patients with ACHD (nϭ50) were studied with cardiac MRI to quantify systemic ventricular volume and function and diffuse fibrosis. The fibrosis index for a single midventricular plane of the systemic ventricle was quantified by measuring T1 values for blood pool and myocardium before and after administration of gadolinium (0.15 mmol/kg) and then adjusted for hematocrit. Results were compared to healthy volunteers (normal controls, nϭ14) and patients with acquired heart failure (positive controls, nϭ4