The aim of this study was to state whether in patients with dilated cardiomyopathy indices of left ventricular function (i.e. left ventricular end-diastolic and end-systolic volumes, ejection fraction, end-systolic pressure to end-systolic volume ratio, mass to volume ratio, elastic stiffness constant) are related to histologic findings, namely percentage of myocytes, of fibrosis and of interstitium. We have studied 26 patients (18 males and 8 females) aged 13-65 years, who underwent right and left heart catheterization, quantitative angiography, coronary angiogram, and in whom a diagnosis of dilated cardiomyopathy was established. All of them had a biopsy specimen taken from the right aspect of interventricular septum, following the technique of King's College. Histology was automatically evaluated with a computerized system for the percent content of myocytes, of fibrous tissue and interstitium. Correlation with hemodynamic parameters was calculated with the least square method. A significant correlation was found between percentage of fibrosis, left and right ventricular ejection fraction and preload index, and between percent myocytes and right ventricular ejection fraction and duration of disease. Thus, left ventricular function indices do not correlate with percent myocytes, as previously shown: this could be due to the fact that alterations in contractile machinery are due primarily to alterations in cell biochemistry. Moreover, the inverse relationship between myocyte content and duration of the disease may reflect those mechanisms, which tend to perpetuate failure in a dilated heart by increasing subendocardial fibrosis. However, the correlation found between myocyte content and right ventricular ejection fraction may be due to the fact that biopsy specimens were taken from the right ventricle.