Young adults have high levels of behavioral health needs and low levels of health insurance coverage.  Access to health insurance for young adults is rising with recent health reforms, but it is unclear how use of hospital-based care, and payment for such care, changes after expanding insurance. To evaluate the association between health insurance coverage expansions and use of hospital-based care among young adults with behavioral health diagnoses. We conducted quasi-experimental difference-in-differences analyses of hospital inpatient and emergency department use from 2003-2009 based on administrative data.  We estimated differential changes in service use among young adults with behavioral health diagnoses in Massachusetts versus other states before and after Massachusetts’ 2006 health reform.    After 2006, uninsurance among 19 to 25 year olds in Massachusetts fell from 25% to 9% (P=.001). Young adults experienced relative declines in inpatient admissions per 1000 for primary diagnoses of any behavioral health disorder (-2.0; P<.001) depression (-0.39; P=.006) and substance use disorder (-1.19; P<.001).  Although emergency department visits with any behavioral health diagnosis rose after 2006, the rise was lower among young adults in Massachusetts compared to Maryland (-14.14 per 1000; P<.001).  Among young adults in Massachusetts, uninsured inpatient admissions and emergency department visits fell by 6.25 and 4.9 percentage points (P=.007 and P=.002) relative to other states.  Expanded health insurance coverage for young adults is not associated with large increases in hospital-based care for behavioral health. Inpatient admissions among young adults with behavioral health diagnoses in Massachusetts declined after 2006 and, relative to other age groups and compared to Maryland, emergency department visits grew much less.  Expanded insurance coverage increased financial protection to young adults with behavioral health diagnoses, and to the hospitals that care for them.