The echocardiographic predictors of ventricular arrhythmias are reported for the Hypertension Arrhythmia Reduction Trial. Men with mild hypertension were withdrawn from their diuretic therapy and repleted with 40 mEq/day of oral potassium and 20 mEq/day of oral magnesium for 1 month. M-mode echocardiography and 24-hour continuous ambulatory electrocardiography were performed on 123 men, mean age 62 years. Forty-eight men (39%) had echocardiographic evidence of left ventricular (LV) hypertrophy defined as an LV mass index greater than 134 g/m2 and this finding was not related to the presence of LV hypertrophy on electrocardiogram or to age. Men who had echocardiographic LV hypertrophy were more likely than men without echocardiographic LV hypertrophy to have greater than or equal to 30 ventricular premature complexes (VPCs)/hr (odds ratio = 2.7; 95% confidence interval = 0.9, 8.0), multiform extrasystoles (odds ratio = 1.7; confidence interval = 0.8, 3.7), episodes of ventricular tachycardia (odds ratio = 2.3; confidence interval = 0.7, 7.1) and the combination of frequent (greater than or equal to 30 VPCs/hr) or complex (ventricular couplets, multiform extrasystoles or episodes of ventricular tachycardia) ventricular arrhythmia (odds ratio = 1.7; confidence interval = 0.8, 3.5). Similar associations between echocardiographic LV hypertrophy and ventricular arrhythmias were observed on 24-hour tracings obtained on entry to the study (before electrolyte repletion) in the 96 men who were taking diuretics at this time. The combination of a frequent or complex arrhythmia was also more common in men aged 60 to 70 compared to men aged 35 to 59 (odds ratio = 3.4; confidence interval = 1.4, 8.2).(ABSTRACT TRUNCATED AT 250 WORDS)