This study aimed to assess the impact of obstructive sleep apnea (OSA) due to adenotonsillar hypertrophy (ATH) on the global myocardial performance in children using tissue Doppler imaging (TDI) and to evaluate the reversibility of the disorder after adenotonsillectomy (AT). The study included 42 children with OSA due to ATH (mean age, 5 ± 3.14 years) as the study group and 45 age- and sex-matched healthy children (mean age, 5.2 ± 3.08 years) as the control group. Polysomnography and echocardiography were performed. Indexed left ventricular mass (LVMi), pulmonary artery systolic pressure, mean pulmonary artery pressure (mPAP), and pulmonary vascular resistance (PVR) were calculated by echocardiography. Tissue Doppler imaging was used to determine the left ventricular and right ventricular myocardial performance index (MPI) of patients and control subjects before and after AT. The patients were classified into mild OSA (apnea-hypopnea index [AHI] 1–5; n = 18)] and moderate to severe OSA (AHI >5; n = 24) according to polysomnography findings. All the children in the control group had an AHI less than 1. They were treated using AT, then reevaluated by polysomnography and echocardiographic examination 6 to 8 months after surgery. Results are described as mean ± standard deviation. The patients with OSA had higher pulmonary artery systolic pressure, mPAP, PVR, LVMi, and right ventricular diastolic diameter than the control subjects. The patients with moderate to severe OSA showed more prominent changes than the patients with mild OSA, but the latter still differed significantly from the control subjects. The TDI-derived right ventricular MPI and left ventricular MPI measurements of the patients with OSA were higher (mean, 0.40 ± 0.08 vs 0.28 ± 0.01; p < 0.001) than those of the control subjects and (0.45 ± 0.05 vs 0.32 ± 0.05; p < 0.001) and correlated well with AHI and mPAP. In addition, mPAP was significantly correlated with AHI. Postoperatively, relief of OSA was validated by polysomnography, and a repeat of the echocardiographic parameters showed no significant differences between the patients and the control subjects. Tissue Doppler imaging can detect the subtle, subclinical changes in cardiac performance that occur in OSA due to adenotonsillar hypertrophy. Such changes generally are reversible after surgical treatment.