Biochemical studies of different groups of patients with sickle β-thalassemia (S-thal) are described. In a group of relatively asymptomatic black patients with sickle thalassemia, there is mild anemia and no clinical symptomatology. α and β-type (βA,βS, and γ) globin chain synthesis in these patients is balanced in bone marrow cells, although there is an excess of α-over β-type chain synthesis in peripheral blood. These mildly affected patients are in contrast to two other groups of S-thal patients of both Mediterranean and black extraction who have anemia and clinical symptomatology and in whom there is absent or more decreased βA-chain production.