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.