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Left ventricular (LV) hypertrophy in adults is a recognized risk factor for the subsequent development of cardiovascular morbidity. To make informed preventive health decisions it is important to understand the interaction of genes and environment on LV mass. In both children and adults, weight is a strong correlate of LV mass. We hypothesized that genetic influences common to both of these variables could in part explain the strong relation between weight and LV mass in children. In a population of 341 twins (11 years old), these questions were asked: (1) How much of the total variance of LV mass is under genetic control? (2) After accounting for weight and weight adjusted for sexual maturity, how much of the remaining variance is genetic? (3) Of the total genetic variance, what proportion is specific for LV mass and what proportion is common to both weight and LV mass? (4) How much of the correlation between these 2 variables is explained by genes common to both LV mass and weight? Univariate genetic analyses documented that genes operating at different magnitudes in boys (63%) and girls (71%) explain a significant proportion of the variance of LV mass. After removing the effect of weight and sexual maturity by regression methods, genes remain an important influence. Bivariate genetic analyses confirmed that genes common to LV mass and weight significantly influence the covariation of these variables and that >90% of the correlation of LV mass and weight is due to common genes. (Am J Cardiol 1991;68:661-668), Left ventricular hypertrophy (enlargement) is generally caused by hypertension, and is a good indication that coronary artery disease will develop. Left ventricular (LV) size increases as body weight increases among adults. In order to make effective recommendations about methods for reducing LV mass, it is necessary to understand the mechanisms regulating it. Because of the association between weight and LV mass, it is likely that genetic factors influence both, but the effects of genetics and the environment are difficult to isolate. A total of 341 pubertal (11-year-old) twins were studied to assess the factors affecting LV mass. The children whose blood pressure was in the upper quintile for their sex and age had greater LV mass. Children in the middle and lower quintiles had normal LV mass. In an earlier study of black twins, the differences in LV mass were greater between dizygotic (fraternal) twins than between monozygotic (identical) twins, and it was concluded that both genetic and environmental factors determined LV mass in black children. In this study univariate genetic analyses demonstrated that genetic influences were responsible for 63 percent of the variance in LV mass in boys and 71 percent in girls. After weight and sexual maturity were removed, genetic factors remained significant. Over 90 percent of the strength of the relation between LV mass and weight was explained by genetic influences. (Consumer Summary produced by Reliance Medical Information, Inc.)