PROJECT ABSTRACT With increasing prevalence of autism spectrum disorder (ASD) in childhood, autistic young persons (10-24 years) are a growing, yet understudied group. The research gap promotes autistic young person’s vulnerability to long-term poor health due to limited understanding of their health care needs and rising pressure on services. Elevated cross-sectional rates of co-occurring conditions and service utilization are well-documented in autistic young persons, but knowledge gaps remain regarding health and services patterns across age, and relevant targets and their timing needed to guide interventions. Further, prior work suggests socioeconomically disadvantaged or race/ethnically diverse groups are particularly at risk for compounded health effects and sub- optimal service patterns. We hypothesize that there are distinct patterns of health conditions and health services across age in autistic young persons which, together, impact future health, and the impact is potentially magnified by socioeconomic disadvantage or race/ethnic disparities. Using national 2008-2023 Medicaid data for a diverse US population aged 10-24 years (1.5 million with 5-years continuous enrollment; 600,000 ASD, 975,000 non-ASD), we will model longitudinal trajectories over a 5-year period of health conditions and services and quantify the combined effect of health and health services history on health over the next 10 years. We will apply group-based multi-trajectory methods to (Aim 1) determine age-specific health conditions trajectory classes (3 domains: brain; physical; behavioral health), and (Aim 2) health services trajectory classes (3 domains: inpatient/emergency; outpatient; medications). We will also quantify impact of health and services class combinations (Aim 3) on study participants’ health in the next 10 years using (a) burden of disease metrics (disability adjusted life years, years of life lived with disability, years of life lost due to premature death, health loss proportion), (b) risk of inpatient/emergency services utilization, and (c) mortality (all-cause mortality; suicide; death due to unintentional injury). Across all Aims, we assess ASD vs. non-ASD overall, as well as heterogeneity by sex, race/ethnicity, and socioeconomic status groups. As part of PHASES’ public health approach, we employ robust, 2-way community engagement methods in all study years for critical input from our community advocates, including autistic individuals, regarding all phases of our research. Our team has the experience and study population knowledge to conduct these novel state-of-the-art trajectory methods and health metrics, in this unique and diverse population based on 15-years of nation-wide Medicaid data. This project will provide critical evidence on multi-domain ‘real world’ health and services experiences and their effects on long term health, needed to optimize health interventions and outcomes for autistic young persons aging into adulthood.