Left ventricular hypertrophy is associated with an increased risk for cardiovascular disease. The known determinants of left ventricular hypertrophy only partially explain its variability. The purpose of this study was to estimate heritability of left ventricular mass. The study sample included adults in the original Framingham Heart Study and the Framingham Offspring Study who were not receiving antihypertensive medications and who were free of coronary heart disease, congestive heart failure, diabetes mellitus, renal insufficiency, valvular heart disease, and severe left ventricular hypertrophy. Intraclass correlations for left ventricular mass among first-degree relatives, second-degree relatives, and unrelated spouse pairs were calculated re determine the contribution of heredity to the variability in left ventricular mass. After adjustments for age, height, weight, and systolic blood pressure, the intraclass correlations between first-degree relatives were .15 (parent-child, P<.001) to .16 (siblings, P<.001), between second-degree relatives the correlation was .06 (P=NS), and between spouses it was .05 (P=NS). The estimated heritability of adjusted left ventricular mass was between .24 and .32. The proportion of the variance in sex-specific left ventricular mass explained by age, height, weight, and systolic blood pressure was .26 in men and .34 in women. On the basis of intraclass correlations for left ventricular mass, incorporation of adjusted left ventricular mass of a parent or sibling would increase the explained variance by an additional .02 to .03. Heredity ex- plains a small, but discernible proportion of the variance in left ventricular mass. Studies are currently under way to identify genetic markers that predict an individual's predisposition to left ventricular hypertrophy. This knowledge may lead to advances in the prevention of left ventricular hypertrophy, which is strongly associated with cardiovascular morbidity and mortality.