People with epilepsy are 10 times more likely to be diagnosed with autism spectrum disorders (ASD) than are people without epilepsy, according to a new nationwide cohort study from Sweden. The population-based study included people with epilepsy (n = 85,201), along with their siblings (n = 80,511) and offspring (n = 98,534). The risk of ASD was also increased in siblings and offspring of individuals with epilepsy. The relationship between epilepsy and ASD was bidirectional: a diagnosis of ASD increased the risk of epilepsy almost fivefold. The findings suggest that disorders have a shared aetiology.