This paper examines the impact of the Massachusetts Health Insurance reform of 2006 on job mobility and employment exit using administrative data from the Social Security Administration. The Massachusetts reform mandated that every resident have insurance coverage and facilitated this initiative by requiring employers to offer coverage, as well as expanding Medicaid and creating health insurance exchanges with subsidized premiums. These elements provided the basis for the Patient Protection and Affordable Care Act (ACA) passed nationwide in 2010, so the experience of workers in Massachusetts provides evidence for how the ACA may affect labor market efficiency. Of particular interest is the extent to which Massachusetts' reform reduced "job lock" – the phenomenon in which workers stay with employers to maintain their health insurance coverage, rather than move to a more productive match at another employer (especially a small firm unlikely to offer coverage) or exit employment entirely. The project measures differential effects by age, gender, and firm size, and tries to disentangle the effects of the employer mandate and the individual mandate by identifying individuals who cross state lines between home and work.