In a large prospective study (1959--1966), the perinatal mortality rate for U.S. Whites was 34 deaths per 1000 total births, for Blacks 51/1000, for Puerto Ricans 41/1000, and for Orientals 23/1000. A number of disorders were responsible for these differences. Premature rupture of the fetal membranes occurred 92 per cent more often in Blacks than in Whites, marked placental growth retardation 56 per cent more often, amniotic fluid infections 45 per cent more often, and major congenital malformations 15 per cent more often. Most other disorders were less common in Blacks than in Whites. Stratifying the data by selected factors, such as prepregnancy body weight and antenatal medical care, eliminated or greatly reduced almost all of these interracial differences. The only major unexplained differences remaining were an excess of amniotic fluid infections and major congenital malformations in Blacks, and an excess of abruptio placentae and large placental infarcts in Whites.