The importance of elevated plasma cholesterol as a risk factor for coronary heart disease has been well established by extensive epidemiological studies. These same studies have shown marked differences in population risks throughout the world. In contrast there is little evidence among populations for variability of plasma cholesterol at birth. During the first months of life there are marked increases in plasma cholesterol and infants apparently begin tracking towards their adult levels. Infants are extremely susceptible during these critical months to dietary and genetic influences on plasma cholesterol. The study of human twins provides a unique opportunity to gain understanding about development of human cholesterol fractions. Comparison of monozygotic and dizygotic twins give estimates of genetic influences, and comparisons of twins with known placental types provide insight into prenatal environmental influences which set cholesterol levels. The present study will extend earlier observations made in this laboratory about sources of variation in plasma cholesterol by studying a cohort of twins from birth to three years of age. Detailed lipid studies will include fractionation of cholesterol by lipoprotein type and into free and esterified fractions. These cholesterol fractions will be correlated with environmental influences including placental types, and diet as well as genetic influencee as estimated by comparing monozygotic with dizygotic twins and twins with the parents. The long term goal is to provide more specific insights into factors affecting plasma cholesterol increases in infancy and to provide clues to methods for preventing increases of cholesterol fractions which in turn increase the risk of atherogenesis.