In recent decades, neonatal and postneonatal mortality rates have declined overall in the United States. Yet, the mortality rates for black infants continue to be approximately twice those for white infants. With the use of data from 45 of the 53 vital statistics reporting areas that participated in the 1980 National Infant Mortality Surveillance project, we extended previous State analyses to describe differences, nationally, in neonatal and postneonatal mortality risks for black and white infants according to gestational age and birth weight. After restricting our analysis to single-delivery infants with known and plausible combinations of gestational age of 26 or more weeks and birth weights of 500 grams (g) or more, the neonatal mortality risk (NMR)--that is, the number of deaths to infants less than 28 days of life per 1,000 live births--for black infants was 1.6 times higher than the NMR for whites. This difference was largely explained by two findings: First, although the NMR was lower for black than for white infants with gestational ages of less than 38 weeks and birth weights less than 3,000 g, that advantage was heavily outweighed by the higher percentage of such births among blacks, accounting for roughly two-thirds of the overall difference in NMR between blacks and whites. Second, most of the remaining difference in NMR was accounted for by higher NMRs among black infants with gestational ages of 38 or more weeks and birth weights of 3,000 g or more. A comparison of the lowest mortality risk for any combination of birth weight and gestational age showed that the black NMR was 1.89 times higher than the white NMR. The postneonatal mortality risk (PNMR)--PNMR equals the number of deaths to infants 28 days to less than 1 year of life per 1,000 neonatal survivors--for black infants was 2.09 times the PNMR for white infants. Black infants had higher PNMRs than white infants for nearly all combinations of birth weight and gestational age. Higher PNMRs among infants with gestational ages of 38 or more weeks and birth weights of 2,500 g or more accounted for 43 percent of the difference in PNMR between black infants and white infants. Eliminating the U.S. black-white infant mortality disparity will require not only reducing the higher frequency of prematurity and low birth weight among black infants, but also improving the survival during both the neonatal and postneonatal periods of term black infants with normal birth weights.