OBJECTIVE: Infant mortality rates (IMR) and neonatal mortality rates (NMR) in the United States have not decreased recently. The purpose of this study was to determine the contributions of birth weight and gestational age subgroups to the IMR and NMR in the United States. METHODS: We used the most recent (1983-2005) US linked birth and infant death data and simple regression analysis to determine the contributions of specifi c birth weight and gestational age subgroups to trends in IMR and NMR. RESULTS: IMR and NMR decreased between 1983 and 2005 for all birth weight and gestational age subgroups. There was an increase in births of very low birth weight infants from 1.2% to 1.5% (P < .001) over this period. The proportion of very low birth weight-infant deaths increased from 42.9% to 54.8%, resulting in recent nonsignificant declines in IMR and NMR. The proportion of live-birth infants <500 g increased from 0.12% to 0.18% (P < .001). The adjusted IMR and NMR over time (excluding infants <500 g) have steeper declining trends than the ones including infants <500 g. The changes in overall IMR and NMR in recent years (2000-2005) are not statistically significant. However, the adjusted IMR and NMR trends during this time are highly significant. CONCLUSIONS: The increased proportions of infants <500 g and other low birth weight infants contribute greatly to the lack of a decrease in IMR and NMR from 2000 to 2005, although birth weight- and gestational age-specific IMR and NMR continue to decrease. Copyright © 2013 by the American Academy of Pediatrics.