The purpose of this study, through a retrospective epidemiological survey carried out over three geographical areas in France, was to characterize the aetiological factors involved in severe mental retardation (SMR) within a geographically defined population of children with disabilities aged between 7 and 16 years. The inclusion criteria for SMR (IQ<50) were met by 1150 children born between 1976 and 1985. Of these children, aetiology was known in 25%, suspected (or unclear) in 26%, and unknown in 49%. These rates of known and suspected aetiology varied between the groups of children with CP and those without CP. An analysis of factors associated with SMR was undertaken among the 144 subjects with SMR, of suspected or unknown aetiology, who had been referred to a neonatal care unit with or without intensive care (NCU) during their neonatal period. These subjects with SMR were compared with 864 children without SMR (control children) who were also referred to an NCU during their neonatal period. The main specific associated factors were a prolonged intubation of more than 24 hours, a very low birthweight (<1500 g) for children with an associated clinical feature of CP, and the presence of isolated neonatal fits and a time of transfer to the NCU of more than 4 hours after birth for children without an associated clinical feature of CP. Although common associated factors were encountered in the children with SMR with CP and the children with SMR without CP, the results of this study suggest differences in the underlying pathogenic factors.