The diseases commonly confused with sickle cell anemia include sickle cell beta-thalassemia in which synthesis of betaA-chains are completely suppressed (HbS-betao-thalassemia). We obtained hematologic measurements and studied globin biosynthesis in five patients with this disorder and compared the results with those obtained in five patients with "mild" sickle cell anemia and seven individuals with sickle cell-beta-thalassemia having hemoglobin A levels of 20-30% (HbS-beta+-thalassemia). A distinction between HbS-betao-thalassemia and sickle cell anemia was not always possible on clinical, hematologic, or electrophoretic grounds. Thalassemia heterozygotes had hypochromia and microcytosis, not generally a feature of sickle cell anemia, although overlap of values did exist. The ratio of alpha to non-alpha, or alpha to betaS-chains in sickle cell anemia approximated unity, whereas patients with HbS-betao-thalassemia had a deficit of beta-chain production relative to that of the alpha-chain. The differentiation of HbS-betao-thalassemia and sickle cell anemia can be best made on the basis of family or biosynthetic study. We estimated the regional prevalence of HbS-betao-thalassemia to be 1:23,000 of the black population.