The Swedish Medical Birth Registry covers virtually all births in Sweden. Gestational age was mainly measured by ultrasonography, but also by the last menstrual period method. Both birth length and birth weight were measured. A linkage between the birth registry and a death registry allows for enumeration of death during the first year of life. The present lecture only presents findings about neonatal mortality. See reading 1, Cheung et al. (2000), for details and findings on perinatal and infant mortality.