We investigated systematically the structure of the myocardium obtained from patients with dilated cardiomyopathy undergoing transplantation because of intractable heart failure. Hearts were explanted at the time of surgery from 12 patients (10 men and 2 women, aged 31-57 years, ejection fraction< 20%) and numerous samples were taken for light and electron microscopy. Biopsies from the left ventricle of 8 patients during operations for atrial septal defect served as control tissue. The most obvious qualitative findings were focal hypertrophy and atrophy of myocytes, enlargement and bizarre shape of nuclei, lack of contractile material and occurrence of numerous small mitochondria. On a quantitative level, the nuclear density was reduced (18%, p< 0.05) but the nuclear profile area was significantly increased (85%, p< 0.001). Thus the nucleus/cytoplasm relationship was altered. The volume density of the contractile filaments was decreased (25%, p< 0.001), but the mitochondrial volume density was unchanged. There was an increase in cell width (39%, p< 0.01) and of the connective tissue content (= fibrosis)(112%, p< 0.001). It is suggested that the nuclear abnormalities may be the primary event in the pathogenesis of dilated cardiomyopathy. These may then lead to a reduced transcriptional rate which most probably is the cause of the lack of myofilaments and other degenerative changes. The deterioration of the structural quality of the hypertrophied myocytes results finally in atrophy and fibrosis and may be the structural correlate of functional disturbances in dilated cardiomyopathy.