The clinical laboratory assumes the paramount role of supplying accurate data to the attending physician for the diagnosis, treatment and prevention of HDN. Maternal prenatal testing identifies patients at risk for Rh-HDN. The antibody titer is of primary value in assessing patients as candidates for amniocentesis. Amniotic fluid analyses provide an assessment of fetal prognosis in HDN and also an assessment of gestational age, lung maturity, and placental function. In severe HDN, amniotic fluid analysis can indicate the need for intrauterine transfusion. Postnatal laboratory studies can confirm the suspected diagnosis of HDN, identify those neonates at risk of developing kernicturus, and provide the physician with information pertaining to the treatment of HDN. Finally, prenatal and postnatal laboratory testing identifies those females eligible for Rh-immune globulin therapy to prevent HDN in subsequent pregnancies.