Kolstad and Kowalski: Evidence from Massachusetts on the Impact of Health Care Reform: [T]he Massachusetts reform gives us a novel opportunity to examine the impact of expansion to near-universal health insurance coverage among the entire state population.... We use a difference-in-difference strategy that compares outcomes in Massachusetts after the reform to outcomes in Massachusetts before the reform and to outcomes in other states.... [R]eform increased insurance coverage among the general Massachusetts population. Our main source of data is a nationally-representative sample of approximately 20% of hospitals in the United States. Among the population of hospital discharges in Massachusetts, the reform decreased uninsurance by 36% relative to its initial level. We also find that the reform affected utilization patterns by decreasing length of stay and the number of inpatient admissions originating from the emergency room. Using new measures of preventive care, we find some evidence that hospitalizations for preventable conditions were reduced... hospital cost growth did not increase after the reform in Massachusetts relative to other states.