Perinatal mortality, birth weight, the distribution of gestational age, and the relationship between gestational age and birth weight have been studied with regard to births reported to the Register of Births during 1976-77. In order to determine the influence of socio-economic factors on the parameters studied, the information from the birth register has been combined with that from the 1975 census in Sweden (including occupation of the mother, income of the family, type of family and housing standard). The total population of 190 024 infants was divided into three groups; 6915 infants (3.6%) were assigned to the most privileged group (group I) and 26 430 (13.9%) to the less privileged group (group III). The perinatal mortality rate did not differ significantly between group I and III. A comparison of the three groups showed a socio-economic difference as regards birth weight and the incidence of preterm as well as of postterm births. The less privileged group has a greater proportion of infants of low birth weight and of infants that were small for gestational age.