Little is known about adult siblings of individuals with autism. We report on cognitive, social and mental health outcomes in 87 adult siblings (mean age 39 years). When younger all had been assessed either as being “unaffected” by autism (n = 69) or as meeting criteria for the “Broader Autism Phenotype” (BAP, n = 18). As adults, all scored within the average range on tests of intelligence, numeracy and literacy. “Unaffected” siblings were functioning well in terms of jobs, independence and social relationships. Levels of social relationships and employment were significantly lower in the BAP group; autism traits and mental health problems were significantly higher. The data suggest that the “broader autism phenotype” is a meaningful concept but more sensitive diagnostic measures are required.