© 2014 Informa Healthcare USA, Inc. All rights reserved. Clinical severity assessment and molecular analysis of β-, α-globin genes and the -158 (C>T) XmnI polymorphism of the Gγ-globin gene were performed in 80 pediatric patients with Hb E (HBB: c.79G>A)/β-thalassemia (β-thal) to investigate the effects of coinheritance of α-thalassemia (α-thal) and other molecular determinants on their clinical severity. The mean age was 9.4±5.1 years. By using clinical severity score, 35 (43.8%), 27 (33.8%) and 18 cases (22.5%) had moderate, mild and severe disease, respectively. Nine β-thal mutations were identified. All were β0 or severe β+ mutations. Five patients (6.3%) had coinherited α0-thal. All five patients had mild disease with baseline hemoglobin (Hb) values of 7.9±1.5g/dL, mild hepatosplenomegaly and close-to-normal growth. Only one required a red blood cell transfusion. The disease severity was significantly different among the groups with and without α-thal (p=0.025), but was not different among the groups with or without the XmnI polymorphism (p=0.071). This study demonstrates that coinheritance of α0-thal alleviates the degree of disease severity in Hb E/β-thal. All our patients with coinherited α0-thal have mild disease.