The aim of this study was to determine sex differences in growth of the heart during puberty. Three-year increments of growth of the heart and body structures and functions related to heart size were compared between the sexes. Echocardiographically estimated left ventricular mass (LVM) represented heart size, body surface area (BSA) represented the area supplied by the left ventricle, and diastolic blood pressure (DBP) represented a segment of arterial resistance. Pubertal changes in these and several other parameters were compared in 67 healthy girls and 84 healthy boys followed longitudinally from 11.5 to 14.5 years. Increments were compared by t-tests. Left ventricular mass of boys was larger than in girls. The sex difference was not significant at 11.5 years, but increased during the three pubertal years. LVM increased 26% in girls and 53% in boys (P <.001). During the same period, BSA increased 22% in girls and 29% in boys (P <.001), while DBP increased 6% in girls and 9% in boys (P <.05). The larger increment of BSA and DBP in boys corresponds to the sex difference in the increment of the heart size. These differences account, in part, for the greater increase (P <.001) in physical working capacity in boys compared to girls. Am. J. Hum. Biol. 9:297-302, 1997. © 1997 Wiley-Liss, Inc.