It has been well known for many years that Black Americans are at increased risk for sickle cell disease and that individuals of Mediterranean ancestry are at increased risk for thalassemias. Beyond this, however, complete assessments of the epidemiology of hemoglobinopathies in Americans have been constrained by lack of large enough sample size, incomplete diagnostic work or testing inadequacies, or missing or misleading ethnicity data. California began universal, mandatory screening of all infants born in the state in 1990, and by January 1996, had screened over 3.3 million infants of various ethnic backgrounds. New information is now available on groups at increased risk for disease, as well as carrier status. The overall prevalence of sickle cell disease (all types) remains high in Blacks at 1 per 396 births, but is lower than expected (from east coast/ Caribbean published data) for Hispanics at 1 m 36,000 births or from previously published data on other higher risk groups such as Middle Eastern (no cases in over 22,000 screens) or Asian Indian (1 case in 16,000 screens). The distribution of different types of sickle cell disease has also changed, with increased numbers of Hb SE disease resulting from multiethnic partnerships. Demographic trends in California have led to a major sift in the at-risk groups for major beta thalassemias, with the majority of cases detected in families of Asian, Southeast Asian, and Asian Indian ancestry. The "new" hemoglobinopathy condition in California, Hb E/Beta. Thalassemia, is found almost exclusively in Southeast Asians with a prevalence of 1 in every 2,600 births. Carrier conditions are found in virtually every ethnic category, with higher than expected rates in non-Hispanic Whites (1 per 600 births).