Explanted hearts were examined to determine whether specific histopathologic features are present in the myocardium of patients with end-stage idiopathic dilated cardiomyopathy (IDC). Extensive histopathologic examination by light microscopy, electron microscopy and immunohistochemistry revealed marked fibrosis in the hearts of 21 of 37 IDC patients and in 26 of 35 patients with heart diseases of known causes. Reactive (interstitial and perivascular) fibrosis predominated in the IDC hearts, whereas both reparative (replacement) fibrosis and reactive fibrosis were found in the comparison group. Endocardial fibroelastosis was found in nine patients with IDC and in 14 patients from the comparison group. Distinct patterns of fibrosis were the sole significant histopathologic difference between myocardial samples from patients with IDC and from those with heart diseases of known causes. The diffuse presence of reactive fibrosis in IDC patients suggests a more generalised dysfunction that affects the composition of the myocardial extracellular matrix.