Distribution of left ventricular (LV) hypertrophy was assessed by wide-angle, 2-dimensional (2-D) echocardiography in 153 patients with hypertrophic cardiomyopathy and compared with the scalar electrocardiogram in the same patients. The most common electrocardiographic alterations were S-T segment changes and T-wave inversion (61%), LV hypertrophy (47%), abnormal Q waves (25%), and left atrial enlargement (24%). LV hypertrophy on the electrocardiogram was significantly more common in patients with the most extensive distribution of LV hypertrophy on 2-D echocardiogram involving substantial portions of both the ventricular septum and LV free wall (type III; 51 of 69, 74%) than in those with more limited distribution of LV hypertrophy (21 of 84, 25%; p less than 0.001). Most patients with hypertrophic cardiomyopathy and normal electrocardiograms (13 of 23) had localized (type I) hypertrophy, but only 4 had the extensive type III pattern of hypertrophy. Abnormal Q waves were significantly more common in those patients without hypertrophy of the anterior, basal septum (type IV; 15 of 27, 56%) than in those with basal septal hypertrophy (23 of 126, 18%; p less than 0.001); abnormal Q waves were uncommon in extensive type III distribution of hypertrophy (13 of 69, 19%). Thus, although no single electrocardiographic abnormality is characteristic of hypertrophic cardiomyopathy, 2-D echocardiography clarifies the significance of certain electrocardiographic patterns: (1) LV hypertrophy on the electrocardiogram, although present in only about half of the study group, was a relatively sensitive (74%) marker for extensive (type III) LV hypertrophy; (2) abnormal Q waves cannot be explained by ventricular septal hypertrophy alone; and (3) a normal electrocardiogram was most commonly a manifestation of localized LV hypertrophy.