Background: Research on the prevalence of autism in Iceland has indicated that one possible explanation of fewer autism cases in older age groups was due to an underestimation of autism in individuals with intellectual disabilities (IDs). The present study systematically searched for autism cases in the adult population of individuals with severe ID living in the city of Reykjavik, Iceland. Methods: Potential participants (n = 256) were recruited through the Regional Office for the Affairs of the Handicapped in Reykjavik. First, a screening tool for autism was applied, followed by the Childhood Autism Rating Scale and finally the Autism Diagnostic Interview-Revised (ADI-R). Results: The point prevalence of severe ID was 3.7/1000 (95% CI 3.2-4.1) with a male-female ratio of 1.2:1. Participation rate in the study was 46.5%. Participants were younger than non-participants and more often residents of group homes. The prevalence of autism was 21% (25/119) (95% CI 14.7-29.2) with a male-female ratio of 1.8:1. Of the individuals with autism, 10/25 (40%) were verbal according to the ADI-R definition, and 18/25 (72%) had active epilepsy and/or other neurological conditions and handicaps. Conclusion: The study identified twice the number of autism cases than those previously recognised within the service system. Autism is a prevalent additional handicap in individuals with severe ID, which should always be considered in this population. There are indications that the estimated prevalence of autism found should be considered minimal.