The first major insurance expansion of the Affordable Care Act – a provision requiring insurers to allow dependents to remain on parents’ health insurance until turning 26 – took effect in September 2010. We estimate this mandate's impacts on numerous outcomes related to health care access, preventive care utilization, risky behaviors, and self-assessed health. We estimate difference-in-differences models with 23–25 year olds as the treatment group and 27–29 year olds as the control group. For the full sample, the dependent coverage provision increased the probabilities of having health insurance, a primary care doctor, and excellent self-assessed health, while reducing body mass index. However, the mandate also increased risky drinking and did not lead to any significant increases in preventive care utilization. Subsample analyses reveal particularly large gains for men and college graduates.