Changes in left ventricular structure and function have been previously described in children with obstructive sleep apnea (OSA). We aimed to determine if these structural and functional cardiac changes are reversible after treatment of OSA with adenotonsillectomy. Children aged 5 to 13 years with OSA and matched healthy controls were recruited. Adenotonsillectomy occurred within 1 month after diagnosis. Echocardiography and polysomnography were repeated postoperatively. Linear mixed models were fitted to echocardiography measures at baseline and follow-up to assess the effect of OSA on cardiac structure and function. These adjusted for age, gender, race, body mass index, systolic, and diastolic blood pressure. The study sample included 373 children, 199 with OSA and 174 healthy controls. In the control group, 114 children completed the study and 112 completed the study in the OSA group. Children with OSA had reduced diastolic function, lower systolic function, and greater left ventricular mass index at baseline compared with healthy controls (all p < 0.05). Measures of active relaxation, elastic recoil and lengthening of the left ventricle impacted overall diastolic function; each of these worsened with increasing OSA severity. Postoperatively, diastolic function improved in children with OSA compared with controls. There were not significant changes in LV mass index or geometry. In conclusion, children with OSA have impaired left ventricular relaxation during diastole indicating early stage diastolic dysfunction. Adenotonsillectomy for OSA signficantly improved diastolic function. Left ventricular remodeling did not change with improvement of OSA.