Abstract The Nordic countries were screened for the occurrence of cases of autism with a same-sexed twin under age 25 years. Twenty-one pairs (11 monozygotic and 10 dizygotic) of twins and one set of identical triplets were found and extensively examined. The concordance for autism by pair was 91% in the monozygotic and 0% in the dizygotic pairs. The corresponding concordances for cognitive disorder were 91% and 30% respectively. In most of the pairs discordant for autism, the autistic twin had more perinatal stress. The results lend support for the notion that autism sometimes has a hereditary component and that perinatal stress is involved in some cases.