In hypertensive adults, first-phase ejection fraction (EF1), a measure of early ventricular contraction is reduced and associated with prolonged systolic contraction and diastolic dysfunction. Whether this is true in children with primary hypertension is unknown. Echocardiography was performed in 47 normotensive and 81 hypertensive children. Hypertensive children were stratified according to tertiles of LVMi (g/m2.7). EF1 was calculated from the fraction of LV volume ejected up to the time of peak aortic flow. E/e' was used as a measure of diastolic function. Myocardial wall stress (MWS) was calculated in a subsample of children from LV volumes and central aortic pressure. Time to onset of relaxation (TOR) was defined as time to peak MWS over ejection time. Normotensive and hypertensive children were of similar age. Hypertensive children in tertiles 2 and 3 of LVMi had higher BMI z-score than normotensives. EF1 was significantly increased in hypertensive children in tertile 1 compared with normotensive children (P < 0.001), whereas in those in tertile 3, it was significantly lower than in normotensive children (P < 0.001). EF1 was negatively associated with LVMi (β = -0.505, P < 0.001), LVM (β = -0.531, P = 0.001) and E/e' ratio (β = -0.409, P < 0.001); in children who had MWS measured, TOR was negatively associated with EF1 (β = -0.303, P = 0.007) and positively associated with E/e' (β = 0.459, P < 0.001). EF1 is preserved or enhanced in hypertensive children with similar LVMi to normotensive children but is increasingly reduced in those with greater LVMi. This reduction of EF1 is associated with prolonged myocardial wall stress and reduced diastolic function.