OBJECTIVE: The purpose of this study was to examine whether postterm gestational age increases the risk of neonatal mortality. STUDY DESIGN: We retrieved data from 1,815,811 liveborn infants in California from 1999 to 2003. We excluded multiple births and congenital anomalies, as well as infants with a gestational age of less than 38 w0d, or greater than 42 w6d, weeks. We used multivariable logistic regression models to adjust for demographic variables thought to confound the association. RESULTS: Compared to infants born at 38, 39, or 40 weeks, those born at 41 w0d to 42 w6d have a greater odds of neonatal mortality (aOR: 1.34, 95% CI, 1.08-1.65). Subdividing by gestational week, infants delivered at 41 w0d to 41 w6d showed elevated mortality relative to earlier term births (aOR: 1.37, 95% CI, 1.08-1.73). Additional analyses support this increased neonatal mortality across all normal birthweight categories. CONCLUSION: Infants born beyond 41 w0d of gestation experience greater neonatal mortality relative to term infants born between 38 w0d and 40 w6d.