We compared the histologic picture of myocardial fiber disarray in thin (4 mu) and thick (25 mu) sections of tissues obtained at autopsy from 18 adults and eight infants with clinically normal hearts, from 10 hearts with concentric hypertrophy (hypertensive patients), nine with myocardial infarction and four with hypertrophic cardiomyopathy (HCM). In the thick sections, so-called bizarre myocardial fiber disarray in thin sections was seen as a bizarre fascicle disarray. Therefore, the usual fiber disarray reported in cases of HCM is actually a fascicle disarray with a three-dimensional complex network. There was no marked difference in distribution and frequency of fascicle disarray among normal adult and infant hearts, and diseased hearts with hypertension and myocardial infarction. In the four hearts with HCM, diffuse bizarre fascicle disarray in the thick section was detected in the septum and anterior and posterior walls of the left ventricles in all cases, and in the lateral walls in one case. In the portions without the diffuse fascicle disarray, the distribution of disarray was the same as that in hearts with no HCM. Such fascicle disarray, including that of HCM, is probably congenital.