IMPORTANCE Enhanced access to preventive and primary care services is a primary focus of the Affordable Care Act, but the potential effect of this law on surgical care is not well defined. OBJECTIVE To estimate the differential effect of insurance expansion on utilization of discretionary vs nondiscretionary inpatient surgery with Massachusetts health care reform as a natural experimental condition. DESIGN, SETTING, AND PARTICIPANTS We used the state inpatient databases from Massachusetts and 2 control states (New Jersey and New York) to identify nonelderly adult patients (aged 19-64 years) who underwent discretionary vs nondiscretionary surgical procedures from January 1, 2003, through December 31, 2010. We defined discretionary surgery as elective, preference-sensitive procedures (eg, joint replacement and back surgery) and nondiscretionary surgery as imperative and potentially life-saving procedures (eg, cancer surgery and hip fracture repair). EXPOSURE All surgical procedures in the study and control populations. MAIN OUTCOMES AND MEASURES Using July 1, 2007, as the transition point between the prereform and postreform periods, we performed a difference-in-differences analysis to estimate the effect of insurance expansion on rates of discretionary and nondiscretionary surgical procedures in the entire study population and for subgroups defined by race, income, and insurance status. We then extrapolated our results from Massachusetts to the entire US population. RESULTS We identified a total of 836 311 surgical procedures during the study period. Insurance expansion was associated with a 9.3% increase in the use of discretionary surgery in Massachusetts (P = .02). Conversely, the rate of nondiscretionary surgery decreased by 4.5% (P = .009). We found similar effects for discretionary surgery in all subgroups, with the greatest increase observed for nonwhite participants (19.9% [P < .001]). Based on the findings in Massachusetts, we estimated that full implementation of national insurance expansion would yield an additional 465 934 discretionary surgical procedures by 2017. CONCLUSIONS AND RELEVANCE Insurance expansion in Massachusetts was associated with increased rates of discretionary surgery and a concurrent decrease in rates of nondiscretionary surgery. If similar changes are seen nationally under the Affordable Care Act, the value of insurance expansion for surgical care may depend on the relative balance between increased expenditures and potential health benefits of greater access to elective inpatient procedures.