Transvenous endomyocardial biopsy by the technique of CAVES and SHUMWAY was performed in 12 patients with the clinical, hemodynamic and angiographic findings of congestive cardiomyopathy (COCM). Coronary artery disease was ruled out by selective arteriography. Light and electron microscopic findings were correlated with a functional classification based on physical working capacity (PWC) and three left ventricular (LV) angiographic parameters (ejection fraction, end-diastolic volume, mean velocity of circumferential fiber shortening). PWC and the LV volumetric indices (vi) were determined in 52 patients and allowed the following grouping: group 1 with normal PWC and borderline VI (subclinical COCM); group 2 with normal PWC and decreased VI; group 3 with reduced PWC and severely depressed VI. Endomyocardial biopsy was successful in 10 out of the 12 patients. The grade of myocardial damage was assessed by 10 different criteria of typical morphologic abnormality. The myocardial damage score correlated closely with functional impairment as present in the three groups. Immunological fixation tests with the biopsy specimens revealed no antigen-antibody complexes. CONCLUSIONS (1) right ventricular endomyocardial samples adequate for morphologic examination are easily obtained by transvenous biopsy; (2) in COCM, functional impairment parallels the extent of morphologic abnormalities although assessments of cardiac function were essentially confined to the left ventricle, whereas the morphologic specimens were obtained from the right ventricle.