To investigate the cardiac reserve of hypertrophic nonobstructive cardiomyopathy (HNCM), an echocardiographic study was performed before and during angiotensin II loading in 22 HNCM patients without left ventricular (LV) systolic dysfunction at rest and 8 normal subjects. LV end-diastolic dimension (LVDd) and time velocity integral (Ie) at aortic annulus were obtained as parameters of preload and forward ejection respectively. Responses of these indices to afterload stress were compared with clinical findings and right ventricular biopsy findings. Based on the reserve of forward ejection characteristics to afterload stress, 22 HNCM patients were divided into 2 groups; Group I: 9 cases with more than 15% decrease of Ie which corresponded to the value of mean - 1SD in normal subjects, Group II: 13 cases with a decrease of Ie less than 15%. There were no significant differences in echocardiographic findings between Group I and II before angiotensin II loading. On the other hand, Group I had a smaller increase of LVDd (p less than 0.05) to afterload stress, a higher incidence of ventricular tachycardia on a Holter electrocardiogram (p less than 0.05), lower maximal oxygen consumption with treadmil exercise test (p less than 0.01), and a larger value of LV end-diastolic pressure (p less than 0.05) than those in Group II, respectively. Although no difference in myocyte diameter was found between the 2 groups, myocardial fibrosis was more severe (p less than 0.001) and the incidence of moderate or severe disarray was higher (p less than 0.05) in Group I than in Group II. Thus, some HNCM patients cannot maintain forward ejection to afterload stress because of impaired preload reserve attributable to a greater stiffness of the left ventricle with severe myocardial lesion.