IMPORTANCE The Affordable Care Act (ACA) completed its second open enrollment period in February 2015. Assessing the law's effects has major policy implications. OBJECTIVES To estimate national changes in self-reported coverage, access to care, and health during the ACA's first 2 open enrollment periods and to assess differences between low-income adults in states that expanded Medicaid and in states that did not expand Medicaid. DESIGN, SETTING, AND PARTICIPANTS Analysis of the 2012-2015 Gallup-Healthways Well-Being Index, a daily national telephone survey. Using multivariable regression to adjust for pre-ACA trends and sociodemographics, we examined changes in outcomes for the nonelderly US adult population aged 18 through 64 years (n = 507 055) since the first open enrollment period began in October 2013. Linear regressions were used to model each outcome as a function of a linear monthly time trend and quarterly indicators. Then, pre-ACA (January 2012-September 2013) and post-ACA (January 2014-March 2015) changes for adults with incomes below 138% of the poverty level in Medicaid expansion states (n = 48 905 among 28 states and Washington, DC) vs nonexpansion states (n = 37 283 among 22 states) were compared using a differences-in-differences approach. EXPOSURES Beginning of the ACA's first open enrollment period (October 2013). MAIN OUTCOMES AND MEASURES Self-reported rates of being uninsured, lacking a personal physician, lacking easy access to medicine, inability to afford needed care, overall health status, and health-related activity limitations. RESULTS Among the 507 055 adults in this survey, pre-ACA trends were significantly worsening for all outcomes. Compared with the pre-ACA trends, by the first quarter of 2015, the adjusted proportions who were uninsured decreased by 7.9 percentage points (95% CI, -9.1 to -6.7); who lacked a personal physician, -3.5 percentage points (95% CI, -4.8 to -2.2); who lacked easy access to medicine, -2.4 percentage points (95% CI, -3.3 to -1.5); who were unable to afford care, -5.5 percentage points (95% CI, -6.7 to -4.2); who reported fair/poor health, -3.4 percentage points (95% CI, -4.6 to -2.2); and the percentage of days with activities limited by health, -1.7 percentage points (95% CI, -2.4 to -0.9). Coverage changes were largest among minorities; for example, the decrease in the uninsured rate was larger among Latino adults (-11.9 percentage points [95% CI, -15.3 to -8.5]) than white adults (-6.1 percentage points [95% CI, -7.3 to -4.8]). Medicaid expansion was associated with significant reductions among low-income adults in the uninsured rate (differences-indifferences estimate, -5.2 percentage points [95% CI, -7.9 to -2.6]), lacking a personal physician (-1.8 percentage points [95% CI, -3.4 to -0.3]), and difficulty accessing medicine (-2.2 percentage points [95% CI, -3.8 to -0.7]). CONCLUSIONS AND RELEVANCE The ACA's first 2 open enrollment periods were associated with significantly improved trends in self-reported coverage, access to primary care and medications, affordability, and health. Low-income adults in states that expanded Medicaid reported significant gains in insurance coverage and access compared with adults in states that did not expand Medicaid.