A population-based survey was conducted among 152,732 Finnish children and adolescents aged under 16 years and living in northern Finland. Diagnoses and associated medical conditions were derived from the hospital and institutional records of this area. One hundred and eighty-seven children with DSM-IV autistic disorder were identified. Associated medical disorders or associated disorders of known or suspected genetic origin were found in 12.3 percent, including tuberous sclerosis, Down syndrome, fragile X syndrome, Klinefelter syndrome, XYY syndrome, chromosome 17 deletion, chromosome 46, XX, dup(8)(p) and mitochondriopathy. Other associated medical disorders identified were epilepsy, hydrocephalus, foetal alcohol syndrome and cerebral palsy. Hearing impairments were found in 8.6 percent and severe impairment of vision in 3.7 percent of the individuals with autistic disorder. Medical disorders seem to have a special impact on the genesis of autistic disorder and need to be thoroughly examined in each child with autistic disorder.