The rapid advances in understanding the genetic and biochemical bases of human hereditary diseases have led to very practical benefits. Increasing numbers of hereditary defects are proving to be responsive to rational management. In addition, the identification of the biochemical or cytogenetic defect in cells cultured from the patient, including fetal cells from amniotic fluid, permits definitive diagnoses to be made on patients even at great distances. This development allows a preventive program for control of about 50 serious genetic diseases through prenatal diagnosis and selective abortion of affected fetuses. High-risk groups for consideration of prenatal diagnosis include parents who have produced a child affected with any one of the above serious hereditary biochemical or chromosomal defects, families carrying X-linked disorders, Ashkenazi Jews with their high frequency of one carrier of the gene for Tay-Sachs disease per 30 individuals, and pregnant women over 40 years of age who produce only