OBJECTIVES: To estimate health care expenditure trends among young adults ages 19-25 before and after the 2010 implementation of the Affordable Care Act (ACA) extended eligibility for dependent private health insurance coverage.  METHODS: Data sources: Nationally representative Medical Expenditure Panel Survey data from 2008 to 2012. Study design: We conducted cross-sectional analyses and employed a difference-in-differences model to estimate health care expenditure trends among young adults ages 19-25 (the treatment group) and ages 27-29 (the control group). Quantile regression was used to capture different associations between the ACA expansion and health care expenditures.  RESULTS: Our results show that the treatment group had 10% lower overall health care expenditures and 21% lower out-of-pocket payment compared to the control group in 2011-2012.The overall reduction was more significant at the higher end of the health care expenditure distribution. Results also show the increased spending on physician visits and prescription drugs and the reduction on inpatient visit costs of the treatment group in 2011-2012. Expenditures of private health insurers and Medicaid were similar before and after the ACA expansion. CONCLUSIONS: Increased health insurance enrollment as a consequence of the ACA extended eligibility for dependent’s private coverage might have successfully reduced spending and catastrophic expenditures.