Infant mortality in San Diego County (USA) in 1988 was 6.9 per 1000 live births. This rate had remained unchanged since 1980. It rose to 8.3 in 1989 and fell to 7.0 in 1990. This study describes the results of an investigation into the causes of infant mortality during 1985. A total of 333 infant deaths were recorded. Nearly half the neonatal deaths were related to perinatal causes and more than half the post-neonatal deaths were attributed to the Sudden Infant Death Syndrome. Two-thirds of all infant deaths occurred in the neonatal period. The contribution of factors in the maternal background and those related to care during labour, as well as postnatally, is described.