We estimate the effect of illicit drug use during pregnancy on two measures of poor infant health: low birth weight and abnormal infant health conditions. We use data from a national longitudinal study of urban parents that includes postpartum interviews with mothers, hospital medical record data on the mothers and their newborns, and information about the neighborhood in which the mother resides. We address the potential endogeneity of prenatal drug use. Depending on how prenatal drug use is measured, we find that it increases low birth weight by 4–6 percentage points and that it increases the likelihood of an abnormal infant health condition by 7–12 percentage points.