It is not known whether birth weight and early childhood growth are associated with the development of cardiac left ventricular mass (LVM) in healthy adolescents. Left ventricular growth and geometric remodeling may have long-term consequences on cardiovascular health later in life. We studied the determinants of LVM and patterns of geometric remodeling in adolescents with specific emphasis on birth size and growth in early childhood. Left ventricular measurements were obtained with echocardiography in 418 adolescents at the age of 15 years in a prospective atherosclerosis prevention study, Special Turku Coronary Risk Factor Intervention Project (STRIP). Birth weight (P=0.0004), current pulse pressure (P=0.013), physical activity level (P=0.0024), weight (P<0.0001), and male sex (P<0.001) had an independent direct association with LVM in adolescents explaining 47% of the variation. Growth in early childhood was not associated with LVM in adolescents. Birth weight (P=0.0066), current weight (P<0.0001), and physical activity level (P=0.0017) were directly associated with left ventricular posterior wall thickness. Current weight was also directly associated with septal thickness (P<0.0001). Boys had a thicker septum than girls (P=0.0092). Normal relative wall thickness and increased left ventricular mass index (eccentric remodeling) (P<0.0001), as well as increase in both variables (concentric, increased LVM) (P=0.0003), were associated with higher body mass index. Our results indicate that birth weight has a long-lasting impact on LVM and normal body weight is beneficial for cardiac structure in adolescents.