The topography and specificity of fibre disarray and fibrosis in hypertrophic obstructive cardiomyopathy were determined in a histological study comprising 40 necropsy hearts--10 with hypertrophic cardiomyopathy, 10 with congestive cardiomyopathy, 10 with aortic valve stenosis, and 10 normal hearts. Seven standard regional sections were sampled from each heart and graded "double-blind" (tissue location and disease entity) for severity and extent of fibre dissarray and four distinct types of myocardial fibrosis. Statistical comparison of the severity and distribution of indices of fibre disarray and fibrosis within each group and between the normal and the disease groups showed that fibre disarray and fibrosis were qualitatively non-specific for hypertrophic cardiomyopathy. However, when fibre disarray was quantified (1) it was significantly increased in hypertrophic cardiomyopathy and allowed separation of hearts with hypertrophic cardiomyopathy from normal hearts and from those with congestive cardiomyopathy and aortic stenosis, (2) it did not vary significantly among sections of the left ventricle (that is, between the septum and the free wall) in hypertrophic cardiomyopathy, (3) it was closely associated with plexiform fibrosis, and (4) it varied independently of wall and septal thickness. Though the histogenesis of fibre disarray is unknown, it probably represents an exaggeration of a non-specific common pathway for many diverse pathophysiological processes.