Passive diastolic properties of the left ventricle were determined in 10 control subjects and 12 patients with dilated cardiomyopathy. Simultaneous left ventricular angiography and high fidelity pressure measurements were performed in all patients. Left ventricular chamber stiffness was calculated from left ventricular pressure-volume and myocardial stiffness from left ventricular stress-strain relations with use of a viscoelastic model. Patients with dilated cardiomyopathy were classified into two groups according to the diastolic constant of myocardial stiffness (beta). Group 1 consisted of seven patients with a normal constant of myocardial stiffness less than or equal to 9.6 (normal range 2.2 to 9.6) and group 2 of 5 patients with a beta greater than 9.6. Structural abnormalities (percent interstitial fibrosis, fibrous content) in patients with dilated cardiomyopathy were assessed by morphometry from right ventricular endomyocardial biopsies. Heart rate was similar in the three groups. Left ventricular end-diastolic pressure was significantly greater in patients with cardiomyopathy (18 mm Hg in group 1 and 22 mm Hg in group 2) than in the control patients (10 mm Hg). Left ventricular ejection fraction was significantly lower in groups 1 (37%) and 2 (36%) than in the control patients (66%). Left ventricular muscle mass index was significantly increased in both groups with cardiomyopathy. The constant of chamber stiffness (beta*) was slightly although not significantly greater in groups 1 and 2 (0.58 and 0.58, respectively) than in the control group (0.35). The constant of myocardial stiffness beta was normal in group 1 (7.0; control group 6.9, p = NS) but was significantly increased in group 2 (23.5). Interstitial fibrosis was 19% in group 1 and 43% (p less than 0.001) in group 2 (normal less than or equal to 10%). There was an exponential relation between both diastolic constant of myocardial stiffness (beta) and interstitial fibrosis (IF) (r = 0.95; p less than 0.001) and beta and fibrous content divided by end-diastolic volume index (r = 0.93; p less than 0.001). It is concluded that myocardial stiffness can be normal in patients with dilated cardiomyopathy despite severely depressed systolic function. Structural alterations of the myocardium with increased amounts of fibrous tissues are probably responsible for the observed changes in passive elastic properties of the myocardium in patients with dilated cardiomyopathy. The constant of myocardial stiffness (beta) helps to identify patients with severe structural alterations (group 2), representing possibly a more advanced stage of the disease.