The aim of this study was to study patterns of contact with hospital for children with autism spectrum disorder (ASD) using Danish population based register data. We included all children born in Denmark from 1994 through 2002. We found that children diagnosed with ASD had an increased rate of contact with hospital, almost regardless of the cause for the hospital contact. Given the overall association between hospital contact for various causes and ASD observed in these data, hospital data should be used cautiously in future studies searching for associations between a specific disease and ASD. If the increased rate of hospital contact overall for children with ASD is not considered, then misleading over interpretations might be made of observed associations between specific diseases and ASD.