IMPORTANCE Young adults have high levels of behavioral health needs but often lack health insurance. Recent health reforms have increased coverage, but it is unclear how use of hospital-based care changed after expanding insurance. OBJECTIVE To evaluate the association between health insurance coverage expansions and use of hospital-based care among young adults with behavioral health diagnoses. DESIGN, SETTING, AND PARTICIPANTS Quasi-experimental analyses of community hospital inpatient and emergency department use from 2003-2009 based on hospital discharge data, comparing differential changes in service use among young adults with behavioral health diagnoses in Massachusetts vs other states before and after Massachusetts' 2006 health reform. This population-based sample included inpatient admissions (n = 2 533 307, representing 12 821 746 weighted admissions across 7 years) nationwide and emergency department visits (n = 6 817 855 across 7 years) from Maryland and Massachusetts for 12- to 25-year-old patients. MAIN OUTCOMES AND MEASURES Inpatient admission rates per 1000 population for primary diagnosis of any behavioral health disorder by diagnosis; emergency department visit rates per 1000 population by behavioral health diagnosis; and insurance coverage for hospital discharges. RESULTS After 2006, uninsurance among 19- to 25-year-old individuals in Massachusetts decreased from 26%to 10% (16 percentage points; 95%CI, 13-20). Young adults experienced relative declines in inpatient admission rates of 2.0 per 1000 for primary diagnoses of any behavioral health disorder (95%CI, 0.95-3.2), 0.38 for depression (95%CI, 0.18-0.58), and 1.3 for substance use disorder (95%CI, 0.68-1.8). The increase in emergency department visits with any behavioral health diagnosis after 2006 was lower among young adults in Massachusetts compared with Maryland (16.5 per 1000; 95%CI, 11.4-21.6). Among young adults in Massachusetts, the percentage of behavioral health discharges that were uninsured decreased by 5.0 (95%CI, 3.0-7.2) percentage points in inpatient settings and 5.0 (95%CI, 1.7-7.8) percentage points in emergency departments relative to other states. CONCLUSIONS AND RELEVANCE Expanded health insurance coverage for young adults was not associated with large increases in hospital-based care for behavioral health, but it increased financial protection for young adults with behavioral health diagnoses and for the hospitals that care for them.