The number and types of people who become eligible for and enroll in the Affordable Care Act's (ACA's) health insurance expansions will depend in part on the factors that cause people to become uninsured for different lengths of time. We used a small-area estimation approach to estimate differences across states in percentages of adults losing health insurance and in lengths of their uninsured spells. We found that nearly 50 percent of the nonelderly adult population in Florida, Nevada, New Mexico, and Texas--but only 18 percent in Massachusetts and 22 percent in Vermont--experienced an uninsured spell between 2009 and 2012. Compared to people who lost private coverage, those with public insurance were more likely to experience an uninsured spell, but their spells of uninsurance were shorter. We categorized states based on estimated incidence of uninsured spells and the spells' duration. States should tailor their enrollment outreach and retention efforts for the ACA's coverage expansions to address their own mix of types of coverage lost and durations of uninsured spells.