The abnormality of the myocardium in the hearts with hypertrophic cardiomyopathy (HC) was assessed from a view point of whether or not the muscle bundle in the midwall layer maintains its normal circular and continuous orientation surrounding the left ventricular (LV) cavity. Examined were 47 autopsied hearts with HC. The LV wall midway between the base and apex was divided into 6 segments in the transverse plane. Histologically, the circular orientation was lacking largely or completely due to marked fascicle disarray in 77% of the anterior and posterior junctional segments. In 33% of the middle portion of the ventricular septum and in 34% of the anterior and posterior portion of the LV free wall, the midwall layer showed disarray of muscle fibers or small fascicles. In contrast, the lateral LV free wall was devoid of disarranged fibers in its midwall layer. Myocardial fibrosis usually was predominant in the portion where disarray was marked. There were deep tissue clefts often in the area of junction. In 11 hearts, 8 of which were from patients older than 65 years, the circular unit was intact in almost every segment, as it was in 9 of 10 control hearts. The destruction of the circular unit in the area of junction in some patients with HC is considered to be a new morphologic aspect of the disease.