Abstract Purpose The objective of this study was to compare two salient neonatal outcomes—respiratory disorders and hyperbilirubinemia—between late-preterm (34–36 weeks) and full-term (37–41 weeks) singleton infants both at the population level and within families. Methods Analyses were based on natality data on all births in the state of New Jersey from 1996 to 2006 linked to newborn hospital discharge records. For population-level models, logistic regression analyses were conducted to estimate unadjusted and adjusted differences in outcomes by gestational age. For within-family analyses, unadjusted and adjusted logistic fixed-effects models were estimated with the latter including factors that differed across births to the same mother. Results Late-preterm birth increased the odds of a neonatal respiratory condition by more than fourfold (odds ratio, 4.08–4.53) and of neonatal hyperbilirubinemia by more than fivefold (odds ratio, 5.11–5.93) even when comparing births to the same mother and controlling for demographic and economic, behavioral, and obstetric factors that may have changed across pregnancies. Conclusions Based on population-level and within-family models, this study provides the strongest evidence to date that late-preterm birth is an important risk factor for adverse neonatal outcomes that other studies have found are associated with cognitive and behavioral disorders in childhood.