Estimating meaningful change thresholds (MCT) on clinical outcome assessments is an important consideration when evaluating treatments. In fragile X syndrome (FXS) research, there has been no consensus on how to define MCT's on several commonly used outcome measures. The purpose of the current study was to determine clinically relevant MCT's of caregiver-rated assessments using data from a phase 3 clinical trials of arbaclofen (Berry-Kravis et al., 2017). Data were collected as a part of previous phase 3, double-blind, placebo-controlled studies of arbaclofen in individuals with FXS (Berry-Kravis et al., 2017). The two studies enrolled age groups of 5-11-years (n = 159) and 12-50-years (n = 119). The current study examines meaningful within-patient change thresholds from baseline to treatment week 8 across several measures: ABC-C FXS ; PSI; Vineland-II; and a Visual Analog Scale (VAS) of Anxiety and Disruptive Behaviors. MCT's were established by using anchor-based methods, using the CGI-S and CGI-I as anchors. Examining the results of the anchor-based analyses and visual CDF plots, MCT's were observed for the pediatric study for the ABC-C FXS subscales (with a range depending on use of CGI-S or CGI-I as anchor): Irritability: 11.1-14.8 points; Hyperactivity: 6.7-8.9 points; and Socially Unresponsive/Lethargic: 6.6-8.1 points; as well both VAS subscales: Anxiety: 28.3-36.2 mm; and Disruptive Behavior: 22.4-27.4 mm. Such thresholds were not observed for the Vineland-II and PSI subscales. Our analysis of MCT's helps set the stage for interpreting clinical trial results in FXS. This may include use of relevant subscales of the ABC-C FXS and VAS as primary outcomes using the MCT's for response definition. This work may help define future study inclusion criteria and enable future interpretation of treatment outcome results in the field., Competing Interests: Declarations. Conflict of interest: Lauren Schmitt is a consultant for Confluence Pharmaceuticals, Stalicla, and Scioto Biosciences with no direct conflicts to the content presented. Elizabeth Berry-Kravis has received funding from Acadia, Alcobra, AMO, Asuragen, Avexis, Biogen, BioMarin, Cydan, Engrail, Erydel, Fulcrum, GeneTx, GW, Healx, Ionis, Jaguar, Kisbee, Lumos, Marinus, Moment Biosciences, Neuren, Neurogene, Neurotrope, Novartis, Orphazyme/Kempharm/Zevra, Ovid, PTC Therapeutics, Retrophin, Roche, Seaside Therapeutics, Taysha, Tetra, Ultragenyx, Yamo, Zynerba, and Vtesse/Sucampo/Mallinckrodt Pharmaceuticals, to consult on trial design or run clinical or lab validation trials in genetic neurodevelopmental or neurodegenerative disorders, all of which is directed to RUMC in support of rare disease programs; Dr Berry-Kravis receives no personal funds and RUMC has no relevant financial interest in any of the commercial entities listed. David Hessl has received funding from the following, all of which are directed to the UC Davis, in support of fragile X treatment programs, and he receives no personal funds and has no relevant financial interest in any of the commercial entities listed: Autifony, Ovid, Tetra/Shionogi, Healx, and Zynerba pharmaceutical companies to consult on outcome measures and clinical trial design. Randall Carpenter is the founder of Seaside Therapeutics, the company that performed the clinical trials. He is also the founder of Allos Pharma, the company that is currently developing arbaclofen for fragile X syndrome. Over the past 24 months, Dejan B. Budimirovic, MD, has received funding from Zynerba Pharmaceuticals (ZYN2-CL-017, ZYN2-CL-033) and Tetra Discovery Partners, Inc. (BPN14770-CNS-204, BPN14770-CNS-301, BPN14770-CNS-302) as an investigator on the listed five clinical trials in individuals with fragile X syndrome (FXS). He has also received funding from CDC (RFA-DD-21–002). All the above funding has been directed to the Kennedy Krieger Institute/the Johns Hopkins Medical Institutions, meaning that Dr. Budimirovic receives no personal funds, and the Institute has no relevant financial interest in any of the commercial entities listed. Paul Wang is an employee of Clinical Research Associates, LLC, a non-profit and formally worked for Seaside Therapeutics. Karen-Walton Bowen has previously worked at Seaside Therapeutics and currently works at Clinical Research Associates, LLC. Craig Erickson is a consultant to Spinogenix, Stalicla, and Scioto Biosciences with no direct conflicts to the content presented., (© 2024. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.)