Purpose – Excessive drinking of fluids occurs across the autism spectrum but despite the detrimental and potentially hazardous consequences very few studies of this phenomenon have been published. Literature on the topic is sparse. The purpose of this paper is to explore the issues via a large on-line survey and a clinic sample. It is hoped this study will encourage further interest in and investigation including exploration of the links with the neurology underlying autistic conditions. Design/methodology/approach – Results were obtained via a large on-line survey of autistic individuals and parents and carers (n=637) and data from referrals to a single diagnostic assessment and diagnostic clinic. Findings – Of 634 respondents of the on-line survey 474 reported excessive drinking of fluids in severe form. Almost two-thirds stated that this started before the age of five years. Of the children and adults seen at the clinic approximately one-third had dunk fluids excessively in the past. The effects of gender, type of autism condition, intellectual disability, reported stress and associated conditions were examined as were the type of fluids drunk and rate of drinking. The response to having to wait for a drink and the occurrence of vomiting and diarrhoea were also examined. Research limitations/implications – This is a preliminary study but with a large sample size. Limitations lie in the sparse amount of literature on this topic as it affects autism and reliance on parental and self-reports from an on-line survey, the majority of whom responded to an appeal for participants for whom excessive drinking of fluids was an established problem. The clinic sample comprised children and adults who had not been referred for reasons connected to excessive drinking of fluids but for whom this was still a significant problem. A further limitation concerns the absence of data on sensory profiles. This would be worth including in any follow up. Practical implications – It is important to be aware of the implications of excessive drinking of fluids on the health and well-being of children and adults on the autism spectrum. As there are potentially lethal consequences associated with such behaviours it is essential that they are recognised, understood and responded to. Social implications – Excessive drinking of fluids has implications for the development of the child and far reaching consequences for physical and social well-being. Originality/value – This is an original paper that draws on the limited literature available but is primarily based on the results of a unique on-line survey and evaluation of a clinic sample.