The myocardium consists of a muscle fibre array surrounded and interspersed by a network of connective tissue, principally collagen, which maintains the functional integrity of the heart. Changes in collagen composition may therefore contribute to altered ventricular function. Collagen composition was examined in cardiac tissue from 15 patients undergoing orthotopic cardiac transplantation. Of these, 10 had severely impaired left ventricular function due to coronary artery disease. The remaining five had dilated cardiomyopathy. Normal heart tissue was taken at autopsy from 25 patients who died of causes unrelated to cardiovascular disease. Left ventricular collagen concentration, estimated from hydroxyproline levels, increased from 48.6 +/- 4.1 mg/g dry weight of tissue in the control group to 95.3 +/- 9.7 mg/g (P less than 0.01) in patients with dilated cardiomyopathy and to 63.5 +/- 9.8 mg/g in the coronary artery disease group. This increase was attributable to an increase in absolute concentrations of both type I and III collagen, determined by separation of cyanogen bromide peptides by sodium dodecyl sulphate polyacrylamide gel electrophoresis. However, there was a significant decrease in the proportion of type III collagen (compared with type I plus III) from 41.8 +/- 1.1% in controls, to 34.6 +/- 1.5% (P less than 0.01) in the coronary artery disease group and 35.8 +/- 2.8% (P less than 0.05) in the dilated cardiomyopathy group. These results suggest that excessive collagen production, with a preponderance of type I, occurs in these forms of myocardial disease, indicative of a remodelling of the collagen matrix, which, by increasing passive myocardial stiffness may contribute to impaired heart function seen in these groups of patients.