This study was set up to compare the frequency of health, educational and behavioural problems in a geographically defined birth cohort of 7-year-old children grouped by weight at birth. The study design was based on a multi-stage postal survey, with sampling stratified by birthweight. It took place in the four counties of Oxfordshire, Buckinghamshire, Berkshire and Northamptonshire which make up the former Oxford region. We studied 1319 live births to women resident in the former Oxford region in 1985, including all those with birthweights under 1500 g, or whose weight was not recorded, and a sample of those who weighed 1500-2499 g, and of those who weighed 2500 g or more at birth. The children in the sample were traced through the National Health Service Central Register (NHSCR) and self-administered questionnaires were sent to their parents, general practitioners (GP) and teachers. Of the 1169 children who were alive at the age of 7 years and were successfully traced, 805 (75%) were followed up by postal survey. The use of health services, and of additional educational support was higher, and school performance was poorer among children who had weighed less than 1500 g at birth than among children who had weighed 2500 g or more, with the rates for children who had weighed 1500-2499 g falling in between. This survey method identifies the higher rate of health and educational problems in children weighing under 2500 g at birth, particularly those with birthweight under 1500 g, compared with other children. The method could be developed to provide a way of monitoring any changes over time in the prevalence of these problems. This information can be used to assess the health and educational needs of 7-year-old children in the population.