The Northern Finland birth cohort comprises 12,058 live births in 1966-96.3% of all births in the region. The investigation was started during pregnancy and the last follow-up of the total series was at the age of 14 years, when the coverage was still large. Smaller samples of the children and data for the study population from national registers were also examined for the older age groups. The health and development of the children was studied, with special emphasis placed on obtaining reliable incidence figures for neurological handicaps and their correlation with perinatal events. In particular, the correlation between low birthweight and handicapping conditions was documented thoroughly. The indicators predictive of low birthweight among the biological characteristics of the mother and the social conditions of the mother and family, included maternal smoking during pregnancy. The latter was associated not only with adverse perinatal outcome but also with reduction in educational achievement and height among survivors.