Awareness and diagnosis of autism in adulthood is on the rise. Studies have considered the impact of receiving an autism diagnosis for parents of children on the spectrum, although only few primarily qualitative studies have considered the self-reported impact of autism diagnosis. The Impact of Diagnosis Scale (IODS) was initially developed with a focus on borderline personality disorder. Our aim was to develop a version suitable for autistic individuals. The research team and a group of autistic advisors revised the IODS items for suitability and accessibility to autistic participants. We gathered participant data for 92 autistic adolescents and adults from the Cooperative Research Centre for Living with Autism (Autism CRC) Study of Australian School Leavers with Autism (SASLA) and the Australian Longitudinal Study of Autism in Adulthood (ALSAA). We used iterated principal factors analysis to explore potential factors, and thematic analysis to explore responses to two open-ended items. Factor analysis suggested three factors of "Service Access (SA)," "Being Understood (BU)," and "Self-Acceptance and Understanding (SU)" for the 12 items of the IODS-Preliminary Revision (IODS-PR). Cronbach's alpha was good overall and acceptable for subdomains. Item mean scores suggest that although impact of autism diagnosis was generally perceived as positive for SU, scores were neutral in other domains. Qualitative analysis identified themes of Self-Understanding, Identity, and Acceptance, Supports and Services, Valence of Response, Relationships, and Camouflaging. The IODS-PR is the first scale to measure the self-reported experience of receiving an autism diagnosis. It showed good psychometrics and provides new insight into the experience of autism diagnosis. Qualitative analysis identified domains that remain unexplored and the potential for an expanded item set. A further revision of the tool will soon be available. It will provide critical information for clinicians and has potential applications for research and service evaluation. Why was this study done?: There are increasing numbers of adults who are only diagnosed with autism in their teen and adult years. Research on this topic is limited, with most using surveys or interviews.What was the purpose of this study?: The purpose was to develop a revision of the Impact of Diagnosis Scale (IODS) to make it suitable to autistic teenagers and adults.What did the researchers do?: We worked with autistic research advisors to create the IODS-Preliminary Revision (IODS-PR), which has 12 items scored on a 7-point agree/disagree scale and two open-ended questions. We then gathered data using the IODS-PR from the Study of Australian School Leavers with Autism (SASLA) and the Australian Longitudinal Study of Autism in Adulthood (ALSAA). We ran a factor analysis on the scores and conducted a thematic analysis of the open-ended responses. One of the autistic advisors reviewed how we interpreted our results.What were the results of the study?: There were 92 autistic participants (46 males, 38 females, 8 nonbinary; mean age of 36 years old). On average, participants were diagnosed with autism at age 30. The factor analysis suggested three domains in the IOD-PR: Self-Acceptance and Understanding, Being Understood, and Service Access. On average, participants' scores suggested receiving an autism diagnosis was helpful for understanding and accepting themselves, but neutral for being understood by others or getting support from services.The thematic analysis identified several themes, the strongest theme was Self-Understanding, Identity, and Acceptance, where participants mostly commented on the positive new self-identity that came from their autism diagnosis. There was a Supports and Services theme that was divided into Enabled Support, Support not needed, and No or poor services. Most concerning was that many participants commented that the autism diagnosis did not enable any access to supports or that there were no appropriate supports available. There was a Valence of Response theme that was divided into Relief, Positive impact, Wish diagnosed earlier, and Negative impact. There was a Relationships theme divided into Connected with autistic community, Improves relationships, and Others lack understanding. Finally, there was a Camouflaging theme.Based on these results, the researchers are working on further revisions to the IODS-PR to make it more useful and accessible.What do these findings add to what was already known?: The adapted IODS shows promise and findings will guide further development of the tool. These early-stage findings agree with what previous research said about the impact of receiving a diagnosis of autism in adulthood.What are potential weaknesses in the study?: There are strengths and weaknesses to using a questionnaire tool to research this topic. Interview research can get a more in-depth understanding of an individual's response to the diagnosis.How will these findings help autistic adults now or in the future?: When the revised IODS is available, it could be used to evaluate support services and help clinicians understand how to help create a more positive response to diagnosis. Our findings confirm more needs to be performed about postdiagnosis supports.