The characteristics ECG findings as they relate to myocardial changes during long-term course of hypertrophic cardiomyopathy (HCM) were studied. Serial changes in ECG were compared with changes left ventriculogram and bi-ventriculogram in 15 patients with HCM without intraventricular conduction disturbance. Serial changes in ECG findings, with special reference to the negative T wave, were categorized in three group; 1) 5 patients with increase or appearance of the negative T wave (A-1 group). 2) 4 patients with decrease or disappearance of the negative T wave (A-2 group). 2) 6 patients without significant changes in the T wave (B-group). A-1 group presented a marked increase in SV₁+RV₅, and of the thickness of anterior and apical wall, and a decrease of the peak dV/dt/EDV (end diastolic volume-normalized peak filling rate), serially. End diastolic volume (EDV) and ejection fraction (EF) did not change significantly. The configuration of left ventriculogram changed from a round form to a spade-like form. A-2 group presented a marked decrease in SV₁+RV₅, the thickness of anterior and apical wall, peak dV/dt/EDV and peak dV/dt/V (left ventricular volume of the time of peak filling-normalized peak filling rote). EF decreased serially, 2 cases of A-2 group presented the clinical picture of dilated cardiomyopathy in the end stage. In B group, SV₂+RV₅, the wall thickness and left ventricular function did not change, serially. In conclusion, serial observations of ECG are useful for assessing alterations in wall thickness, LV-form and LV-function.