Autism spectrum disorder and attention deficit/hyperactivity disorder overlap in several ways, raising questions about the nature of this comorbidity. Rommelse et al. published an innovative review of candidate endophenotypes for autism spectrum disorder and attention deficit/hyperactivity disorder in cognitive and brain domains. They found that all the endophenotypic impairments that were reviewed in attention deficit/hyperactivity disorder were also present in autism spectrum disorder, suggesting a continuity model with attention deficit/hyperactivity disorder as "a light form of autism spectrum disorder." Using existing data, 75 adults with autism spectrum disorder and 53 with attention deficit/hyperactivity disorder were directly compared on autistic symptoms with the autism spectrum quotient, and on the endophenotypic measure of temperament and character, using the Abbreviated (Dutch: Verkorte) Temperament and Character Inventory. Based on the hypothesis that attention deficit/hyperactivity disorder and autism spectrum disorder are disorders on a continuous spectrum, autism spectrum quotient scores and abbreviated Temperament and Character Inventory scores were expected to be different from normal controls in both disorders in a similar direction. In addition, the autism spectrum quotient and abbreviated Temperament and Character Inventory scores were expected to be closely correlated. These conditions applied to only two of the seven Abbreviated Temperament and Character Inventory scales (harm avoidance and self-directedness), suggesting that temperament and character as an endophenotype of autism spectrum disorder and attention deficit/hyperactivity disorder provides only partial support for the continuity hypothesis of autism spectrum disorder and attention deficit/hyperactivity disorder.