["Journal of Intellectual Disability Research, EarlyView. ", "\nABSTRACT\n\nBackground\nDown syndrome regression disorder (DSRD) is a rare phenomenon of regression impacting individuals with Down syndrome (DS), occurring in adolescence to early adulthood and characterized by acute‐to‐subacute deterioration of functioning. Cognitive and behavioural phenotypes associated with DSRD have been examined using physician checklists but not yet through direct assessments or caregiver questionnaires. The goal of this initial study was to explore the cognitive and behavioural features of those with DS and regression (DS+regression) through direct assessment and informant questionnaires.\n\n\nMethods\nA total of 25 individuals (DS+regression n = 13 and DS n = 12) participated in the study. Participants were administered a standardized measure of intellectual functioning, and caregivers completed questionnaires assessing executive functioning, mood, and adaptive functioning.\n\n\nResults\nThe two groups did not significantly differ on a standardized measure of intellectual functioning, although these results were based upon a reduced sample size (DS+regression n = 8), which limited interpretability. Results tentatively suggest that caregivers of those with DS+regression endorsed concern for executive functioning, mood and adaptive functioning in comparison to those without regression.\n\n\nConclusions\nThis study provides emerging evidence of cognitive and behavioural features reported by caregivers that may be associated with DSRD, with the goal of informing future studies with larger sample sizes. Our findings also emphasize the importance of evaluations for patients with DSRD, and the utility of caregiver‐report measures to identify symptoms of regression for treatment planning.\n\n"]