One hundred and seventeen boys (10, 12, and 14 yr of age) were studied to determine the relation between maximal gas transport capacity (VO2 max) and cardiac size and the interaction of growth and physical activity on this relation. VO2 max was measured during treadmill exercise and heart size was obtained during supine rest by M-mode echocardiography. Left ventricular end-diastolic diameter (LVIDd), resting stroke volume (SV), and calculated left ventricular mass (LVM) were highly correlated (0.75 < r < 0.84) with VO2 max within all age categories. Subjects with high VO2 max had significantly (P < 0.05) larger LVIDd, end-diastolic volumes, resting SV, calculated LVM, and, with the exception of the 12 yr olds, larger left ventricular end-systolic diameters and volumes than subjects with lower VO2 max. Multiple linear regression analysis revealed that fat-free weight accounted for most of the variance in VO2 max and that cardiac dimensions were only of minimal importance in determining maximal gas transport capacity in this study. The seemingly good relationship between cardiac dimensions and VO2 max was mainly attributable to the shared influence of body size on both these factors.