Cross sectional echocardiograms were recorded within one week of death in seven patients with valvular heart disease, four with coronary artery disease, and nine with congenital heart disease. Regional echo amplitude was measured from the cross sectional display by constructing histograms of pixel intensity. Parietal pericardium was used as an internal standard for setting the gain of the instrument. At necropsy myocardium was taken from the free wall of the left ventricle, the papillary muscles, and the septum. Fibrosis was assessed histologically and biochemically as hydroxyproline content. In individual samples histological and biochemical estimates were correlated. In all regions other than the septum in patients with left ventricular hypertrophy, log [collagen] correlated with median pixel intensity. The amplitude of reflected echoes from the hypertrophied septum was significantly higher than that from other samples but was similarly correlated with collagen content. Agreement between echo amplitude and histological grade was significantly less good. Thus in chronic left ventricular disease myocardial collagen content appears to be the major determinant of regional echo intensity. Reproducibility of measurements and more rigorous definition of tissue abnormalities will, however, require further study.