ABSTRACT Obstructive sleep apnea syndrome (OSAS) is highly prevalent in people with Down syndrome (DS), but few studies have investigated its emergence, evolution, and contribution to patient-reported outcomes and co- morbidities, including neurocognitive dysfunction and cardiometabolic and cardiovascular risk. These knowledge gaps have stymied DS health care recommendation development as highlighted by the American Academy of Pediatrics guidelines that all children with DS undergo a sleep study by 4 years of age but that do not address ongoing surveillance. Moreover, major challenges to both clinical care and research related to OSAS in DS is the requirement of an in-laboratory overnight polysomnography (PSG) partly due to the scarcity of dedicated sleep laboratories with expertise in the care of individuals with intellectual disabilities. This supplement will leverage an ongoing study of feasibility, acceptability, and performance of home sleep apnea testing (HSAT) vs PSG in 35 youth aged 10-20 years with DS and expand enrollment by n=15 additional individuals aged 16-30y to 1) secure training in neurocognitive testing in young adults with DS through a new collaboration, 2) estimate recruitment yields when fasting blood work and neurocognitive testing are included in the study, and 3) obtain preliminary estimates examining the relationships of OSAS with a) neurocognitive function and b) cardiometabolic risk markers and OSAS treatment uptake in individuals with OSAS identified in the context of the parent award. This work will inform an OSAS intervention study aimed at adult comorbidities (DS-related Alzheimer disease and dementia, cardiovascular health) and a longitudinal history study of OSAS in adolescents and young adults with DS. Importantly, data derived from this supplement will ultimately address knowledge gaps surrounding OSAS screening, patient-focused and clinical relevance of OSAS treatment, and potentially modifiable risk factors for dementia.