Diffuse myocardial fibrosis is a final end point in most cardiac diseases. It is missed by the cardiovascular magnetic resonance (CMR) late gadolinium enhancement technique. Currently, quantifying diffuse myocardial fibrosis requires invasive biopsy, with inherent risk and sampling error. We have developed a robust and noninvasive technique, equilibrium contrast CMR (EQ-CMR) to quantify diffuse fibrosis and have validated it against the current gold standard of surgical myocardial biopsy. The 3 principles of EQ-CMR are a bolus of extracellular gadolinium contrast followed by continuous infusion to achieve equilibrium; a blood sample to measure blood volume of distribution (1-hematocrit); and CMR to measure pre- and postequilibrium T1 (with heart rate correction). The myocardial volume of distribution is calculated, reflecting diffuse myocardial fibrosis. Clinical validation occurred in patients undergoing aortic valve replacement for aortic stenosis or myectomy in hypertrophic cardiomyopathy (n=18 and n=8, respectively). Surgical biopsies were analyzed for picrosirius red fibrosis quantification on histology. The mean histological fibrosis was 20.5+/-11% in aortic stenosis and 17.1+/-7.4% in hypertrophic cardiomyopathy. EQ-CMR correlated strongly with biopsy histological fibrosis: aortic stenosis, r(2)=0.86, Kendall Tau coefficient (T)=0.71, P<0.001; hypertrophic cardiomyopathy, r(2)=0.62, T=0.52, P=0.08; combined r(2)=0.80, T=0.67, P<0.001. We have developed and validated a new technique, EQ-CMR, to measure diffuse myocardial fibrosis as an add-on to a standard CMR scan, which allows for the noninvasive quantification of the diffuse fibrosis burden in myocardial diseases.