Cardiovascular complications are well recognized in β-thalassemia and sickle cell disease (SCD). The objective of this study was to evaluate left ventricular (LV) structural and functional changes and their relationship to sleep-disordered breathing (SDB) in children with β-thalassemia and SCD. One hundred patients recruited from the hematology clinic were subjected to Pittsburgh Sleep Quality Index score; 26 patients had positive score (Pittsburgh Sleep Quality Index ≥5) (15 β-thalassemia major and 11 SCD) and were compared with 25 age-matched and sex-matched controls. All underwent polysomnography and tissue Doppler echocardiography. SDB was detected in 73% of thalassemia patients (all had increased LV mass index [LVMI], diastolic dysfunction [increased E/Em], and 53% had pulmonary hypertension [tricuspid valve resurgence (TR) velocity ≥2.5 m/s]) and in 46% of SCD patients ( all had increased LVMI, 81.8% had pulmonary hypertension, and 76% had diastolic dysfunction). Sleep O2 saturation of β-thalassemia patients negatively correlated with TR velocity and LVMI (P=0.027, 0.015), and lower asleep O2 saturation was associated with increased E/Em. In SCD patients, sleep and awake O2 saturation negatively correlated with TR velocity and E/Em (P=0.024 and 0.041), and lower sleep O2 saturation was associated with increased LV diameter (P=0.021). SDB is common and associated with LV structural and functional changes in β-thalassemia and SCD.