Improvement in α:non-α ratio and consequent decrease of free α-globin chain might be the cause of beneficial effects of hydroxyurea therapy. Two patients who felt better didn't show significant increase in their fetal hemoglobin level, and this is in contradiction with the hypothesis claiming that the HbF level increase is the cause of such therapeutic effect. In spite of the unclear mechanism of action of this drug, hydroxyurea therapy had noticeable impacts on thalassemia intermedia and also sickle cell disease and even patients suffering from thalassemia major.