Congestive heart failure with dilated left ventricle developed in two patients with symptomatic hypertrophic obstructive cardiomyopathy. Both patients previously underwent cardiac surgery for relief of their outflow obstruction. Alterations in structure and function of the left ventricle during their episode of cardiac failure and thereafter were documented by echocardiography. The findings suggest that progression to left ventricular dilatation is a potential complication in patients with hypertrophic obstructive cardiomyopathy.