Parents of children with autism spectrum disorders (ASD) use a wide range of interventions including poorly evidenced dietary interventions. To investigate parents' and professionals' experience of dietary interventions and attitudes towards a proposed trial to evaluate the gluten free casein free diet (GFCFD). Survey of U.K. parents of children with ASD, and professionals. 258 parents and 244 professionals participated. 83 of children had received a range of dietary manipulations; three quarters of professionals have been asked for advice about GFCFD. Respondents identified an inadequate evidence base for dietary interventions in ASD and suggested modifications to a proposed trial design. Both parents and professionals supported the need for further evaluation of dietary interventions in ASD.