This study was undertaken to determine whether the occurrence of ventricular tachycardia (VT) in patients with hypertrophic cardiomyopathy (HC) is related to the magnitude and extent of left ventricular (LV) hypertrophy. Extent of LV hypertrophy was assessed using 2-dimensional echocardiography in 30 patients with HC in whom VT had been documented on 24-hour ambulatory electrocardiographic (ECG) monitoring, and the extent of LV hypertrophy in these patients was compared with that of a control group of 61 patients with HC who had normal ambulatory ECG recordings. Severe LV hypertrophy, involving at least 3 of the 4 LV segments, occurred significantly more often in patients with documented VT (16 of 30, 53%) than in those with normal ambulatory ECG findings (13 of 61, 21%; p less than 0.002). Conversely, mild LV hypertrophy, involving only 1 LV segment, occurred significantly less often in patients with VT (5 of 30, 17%) than in the control subjects (32 of 61, 52%; p less than 0.001). Moderate LV hypertrophy, involving 2 of the 4 LV segments, occurred about as frequently in patients with VT (9 of 30, 30%) as in patients with normal ambulatory ECG findings (16 of 61, 26%; p greater than 0.05). In addition, the LV wall thickness index, a quantitative measure of overall extent of LV hypertrophy, was also significantly higher (thereby indicating a greater magnitude of hypertrophy) in patients with documented VT (72 +/- 17 mm) than in those with normal ambulatory ECG recordings (61 +/- 14 mm; p less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)