Idiopathic dilated cardiomyopathy (IDC) is characterized by left ventricular (LV) enlargement with systolic dysfunction, other causes excluded. When inherited it represents familial dilated cardiomyopathy (FDC). We hypothesized that IDC or FDC would show with cardiac magnetic resonance (CMR) increased myocardial accumulation of gadolinium contrast at steady state and decreased baseline myocardial blood flow, due to structural alterations of the extracellular matrix compared to normal myocardium. CMR was performed in 9 persons affected with IDC/FDC. Healthy controls came from the general population (N=6), or were unaffected family members of FDC patients (N=3), without signs or symptoms of IDC/FDC or any structural cardiac abnormalities. The myocardial partition coefficient for gadolinium contrast (_Gd) was determined by T1 measurements. Left ventricular (LV) shape and function, and myocardial blood flow (MBF) were assessed by standard CMR methods. _Gd was elevated in IDC/FDC patients versus healthy controls (_Gd = 0.56±0.15 vs. 0.41±0.06; p=0.002), and correlated with LV enlargement (r=0.61 for _Gd vs. EDV indexed by height, p<0.01), and with ejection fraction (r=-0.80; p<0.001). The extracellular volume fraction was higher in IDC patients than in healthy controls (0.31±0.05 vs. 0.24±0.03; p=0.002). Resting MBF was lower in IDC patients (0.64±0.13 vs. 0.91±0.22, p=0.01) than unaffected controls, and correlated with both the partition coefficient (r=-0.57; p=0.012) and the extracellular volume fraction (r= 0.56, p=0.019). The expansion of the extracellular space correlated with reduced myocardial blood flow and ventricular dilation. Expansion of the extracellular matrix may be a key contributor to contractile dysfunction in IDC patients.