A precise assessment of left ventricular (LV) function was performed in 20 patients with hypertrophic cardiomyopathy (HCM) and 14 patients with congestive cardiomyopathy (CCM) to elucidate the pathophysiology of these diseases, expecting to establish a fundamental therapeutic approach of them. Twenty-two patients who underwent cardiac catheterization and were found to have normal LV function served as normal control. LV preload, defined as LV end-diastolic stress, was normal in HCM, and it was elevated in patients with CCM and congestive heart failure (CHF). While an index of afterload, defined as mid-systolic stress, was markedly reduced in HCM, it was elevated in CCM and CHF. Although three indices of LV contractility including the rate of LV pressure rise divided by developed pressure of 40 mmHg ((dp/dt)/DP40), ejection fraction (EF) and LV minute work index, were all in a normal range in HCM, they were very low in CCM. This observation indicates that the systolic function of HCM is normal as a pump, and that it is markedly reduced in CCM. One index of LV relaxation, which was the peak rate of LV pressure fall (peak negative dp/dt) was reduced in both HCM and CCM, and the other index, which was the time constant of LV pressure fall (the time constant T) was prolonged in both of these diseases. An index of compliance, defined as diastolic elastic stiffness constant (K) was high in HCM and normal in CCM, and the other index, defined as end-diastolic elastic stiffness was normal in HCM and high in patients with CCM and CHF. This observation suggests that chamber compliance is low in HCM, and that muscle compliance begins to decline with the appearance of CHF in CCM. Although poor systolic function is evident in CCM, poor contractility or inadequate contraction of a unit muscle is also suspected in HCM since 1) normal EF was maintained with very low afterload, 2) LV end-systolic volume index was normal with very low end-systolic stress, and 3) LV unit muscle minute work about one half of normal value with normal preload. Several important therapeutic guidelines can be derived from this study: As CCM is a disease of reduced LV contractility, the main therapy for this disease is to enhance the contractility. Elevation of preload and afterload are associated with the appearance of CHF, and this coincides with New York Heart Association functional class III. Therefore, the indication of preload and/or afterload reducing agents, such as diuretics and vasodilators, is considered for such patients. Although preload and/or afterload reducing agents are well known to be contraindicated in obstructive type of HCM, since they increase the intracardiac pressure gradient, these agents must be used with great precaution in nonobstructive type, as they reduce preload suddenly, and life-threatening low cardiac output might take place. For the same reason, strenuous exercise must be prohibited in HCM...