Windkessel function is governed by conductance artery compliance that is associated with cardiovascular disease in adults independently of other risk factors. Sex-related differences in conductance artery compliance partly explain the sex-related differences in risk of cardiovascular disease. Studies on sex-related differences in conductance artery function in prepubertal children are few and inconclusive. This study determined the conductance artery compliance and cardiac function by magnetic resonance imaging in 150 healthy children (75 girls) aged 7-10 yr. Any sex-related difference in conductance artery function was determined with correction for other potential predictors in multivariable linear regression models. Our data showed that ascending [crude mean difference 1.11 95% confidence interval (CI) (0.22; 2.01)] and descending [crude mean difference 1.10 95% CI (0.09; 1.91)] aortic distensibility were higher in girls, but differences disappeared after adjustment for pubertal status and other identified potential predictors. Systolic and diastolic blood pressure, cardiac output, left ventricle (LV) systolic function, and total peripheral resistance did not differ between the sexes. In girls, heart rate was 7 beats/min higher, whereas pulse pressure (by 2 mmHg), LV end-diastolic volume index (by 7 mL), and stroke volume (by 5 mL) were lower. LV peak filling rate indexed to LV end-diastolic volume was 0.5 s_1 higher in girls. In conclusion, prepubertal girls and boys have equal conductance artery function. Thus, the well-known sex difference in adult conductance artery function seems to develop after the onset of puberty with girls initially increasing aortic distensibility.