Accessible Summary We wanted to know what care staff felt about difficult behaviour displayed by people with autism, learning disabilities or Down syndrome. We asked many care staff who worked with people with disabilities to watch a video. The video was about a made up lady called Sophie. Sometimes Sophie was described as having autism, other times as having learning disabilities and sometimes as having Down syndrome. After staff members watched the video, we asked them how they felt about Sophie's difficult behaviour. Staff viewed Sophie's behaviour better when she was described as having autism and worst when she was described as having learning disabilities. We believe that staff should view all people equally regardless of the way they are described. We have used the information about this study to make recommendations about staff training. Abstract Background: This study investigated whether care staff's causal attributions and emotional reactions to the challenging behaviour displayed by service users were influenced by the service user's diagnostic label. Materials and Method: One hundred and twenty care staff were randomly allocated to one of three conditions. Participants viewed a video of a senior staff member describing a service user, varying only in diagnostic label (autism, learning disability or Down syndrome). Participants then rated their endorsement of possible causes and emotional reactions to challenging behaviour. Results: Participants in the Autism and Down syndrome groups made more use of biomedical causes and less use of learned behaviour as an explanation for challenging behaviour than those in the Learning Disabilities group. Those in the former groups reported more positive and fewer negative emotions than those in the Learning Disabilities group. Conclusions: The way staff viewed people with learning disabilities was affected by their diagnostic label. Implications for further research and training have been discussed.