In a prospective community-based study of 103 children with febrile convulsions (FC), social and genetical factors were compared with 193 age and sex matched referents sampled from the community. FC were found more often among parents of cases than referents (odds ratio 21.0; p less than 0.001). A history of FC in any type of relative was found in 39.8% of cases and 10.4% of referents (odds ratio 5.8; p less than 0.001). The familial occurrence of FC is best described by a multifactorial (polygenic) model. No difference was found between groups in the occurrence of other diseases than FC affecting the central nervous system and only one case was mentally retarded. No difference was found between cases and referents in the age of independent walking and ability to speak at age 18 months. Children with FC more often had infectious diseases than referents (odds ratio 2.84; p = 0.002). Children with FC more often lived in apartments and spent their time in day by day care institutions (odds ratio 2.91; p = 0.02). Parents' occupation did not differ between groups. Mothers of children with FC more often had compulsory education only compared with mothers of referents.