Objective: Insurance coverage for young adults has increased since 2010, when the Affordable Care Act (ACA) required insurers to permit children to remain on parental policies until age 26 as dependents. This study estimated the association between the dependent coverage provision and changes in young adults' use of hospital-based services for substance use disorders and non-substance use psychiatric disorders. Method: The authors conducted a quasi-experimental comparison of a national sample of non-childbirth-related inpatient admissions to general hospitals (a total of 2,670,463 admissions, 430,583 of which had primary psychiatric diagnoses) and California emergency department visits with psychiatric diagnoses (N=11,139,689), using data spanning 2005 to 2011. Analyses compared young adults who were targeted by the ACA dependent coverage provision (19-to 25-year-olds) and those who were not (26-to 29-year-olds), estimating changes in utilization before and after implementation of the dependent coverage provision. Primary outcome measures included quarterly inpatient admissions for primary diagnoses of any psychiatric disorder per 1,000 population; emergency department visits with any psychiatric diagnosis per 1,000 population; and payer source. Results: Dependent coverage expansion was associated with 0.14 more inpatient admissions for psychiatric diagnoses per 1,000 for 19-to 25-year-olds (targeted by the ACA) than for 26-to 29-year-olds (not targeted by the ACA). The coverage expansion was associated with 0.45 fewer psychiatric emergency department visits per 1,000 in California. The probability that inpatient admissions nationally and emergency department visits in California were uninsured decreased significantly. Conclusions: ACA dependent coverage provisions produced modest increases in general hospital psychiatric inpatient admissions and higher rates of insurance coverage for young adults nationally. Lower rates of emergency department visits were observed in California.