A pattern of left ventricular hypertrophy evident on the electrocardiogram is a harbinger of morbidity and mortality from cardiovascular disease. Echocardiography permits the noninvasive determination of left ventricular mass and the examination of its role as a precursor of morbidity and mortality. We examined the relation of left ventricular mass to the incidence of cardiovascular disease, mortality from cardiovascular disease, and mortality from all causes in 3220 subjects enrolled in the Framingham Heart Study who were 40 years of age or older and free of clinically apparent cardiovascular disease, in whom left ventricular mass was determined echocardiographically. During a four-year follow-up period, there were 208 incident cardiovascular events, 37 deaths from cardiovascular disease, and 124 deaths from all causes. Left ventricular mass, determined echocardiographically, was associated with all outcome events. This relation persisted after we adjusted for age, diastolic blood pressure, pulse pressure, treatment for hypertension, cigarette smoking, diabetes, obesity, the ratio of total cholesterol to high-density lipoprotein cholesterol, and electrocardiographic evidence of left ventricular hypertrophy. In men, the risk factor-adjusted relative risk of cardiovascular disease was 1.49 for each increment of 50 g per meter in left ventricular mass corrected for the subject's height (95 percent confidence interval, 1.20 to 1.85); in women, it was 1.57 (95 percent confidence interval, 1.20 to 2.04). Left ventricular mass (corrected for height) was also associated with the incidence of death from cardiovascular disease (relative risk, 1.73 [95 percent confidence interval, 1.19 to 2.52] in men and 2.12 [95 percent confidence interval, 1.28 to 3.49] in women). Left ventricular mass (corrected for height) was associated with death from all causes (relative risk, 1.49 [95 percent confidence interval, 1.14 to 1.94] in men and 2.01 [95 percent confidence interval, 1.44 to 2.81] in women). We conclude that the estimation of left ventricular mass by echocardiography offers prognostic information beyond that provided by the evaluation of traditional cardiovascular risk factors. An increase in left ventricular mass predicts a higher incidence of clinical events, including death, attributable to cardiovascular disease.