This study aimed: to assess the clinical value of endomyocardial biopsy (EMB) in 76 patients with dilated cardiomyopathy (DCM); to look for residual evidence of myocarditis, especially in patients in whom EMB was undertaken shortly after the onset of cardiac symptoms; to identify ultrastructural changes which may indicate the cause of DCM; and to see if the observed ultrastructural alterations correlated with duration of symptoms, left ventricular ejection fraction and the ratio of left ventricular wall thickness to chamber diameter. Biopsy diagnosis of DCM was possible with adequate exclusive clinical data combined with evidence of myofibre hypertrophy and exclusion of other morphologically detectable causes of cardiac hypertrophy and dilatation, e.g. myocarditis. The ultrastructural features did not correlate with duration of symptoms, ejection fractions or ratio of left ventricular wall thickness to chamber diameter. The lack of evidence of myocarditis in EMB specimens from our DCM patients, including those with a short history, suggests that myocarditis is an uncommon cause of DCM. Only a quarter of the biopsy specimens showed fibrosis and the ultrastructural features were those of hypertrophy. Degenerative changes were present in varying percentages of the specimens examined (e.g. myofilament loss in 64%). This study suggests that conditions other than myocarditis should be considered as causes of DCM--such causes (e.g. metabolic dysfunction) may be potentially reversible if recognized and treated.