IMPORTANCE Health insurance coverage is dynamic in the United States, potentially changing from month to month. The Patient Protection and Affordable Care Act (ACA) aimed to stabilize markets and reduce financial burden, particularly among those with preexisting conditions. OBJECTIVE To describe the risks of insurance churn (ie, gain, loss, or change in coverage) and catastrophic health expenditures among nonelderly patients with cancer in the United States, assessing for changes associated with ACA implementation. DESIGN, SETTING, AND PARTICIPANTS This retrospective, cross-sectional study uses data from the Medical Expenditure Panel Survey, a representative sample of the US population from 2005 to 2018. Respondents included were younger than 65 years, identified by health care use associated with a cancer diagnosis code in the given year. Statistical analysis was conducted from July 30, 2020, to January 5, 2021. EXPOSURES The Patient Protection and Affordable Care Act. MAIN OUTCOMES AND MEASURES Survey weights were applied to generate estimates for the US population. Annual risks of insurance churn (ie, any uninsurance or insurance change or loss) and catastrophic health expenditures (spending >10% income) were calculated, comparing subgroups with the adjusted Wald test. Weighted multivariable linear regression was used to assess for changes associated with ACA implementation. RESULTS From 6069 respondents, we estimated a weighted mean of 4.78 million nonelderly patients (95% CI, 4.55-5.01 million; female patients: weighted mean, 63.9% [95% CI, 62.2%-65.7%]; mean age, 50.3 years [95% CI, 49.7-50.8 years]) with cancer annually in the United States. Patients with cancer experienced lower annual risks of insurance loss (5.3% [95% CI, 4.5%-6.1%] vs 7.6%[95% CI, 7.4%-7.8%]) and any uninsurance (14.6% [95% CI, 13.3%-16.0%] vs 24.1% [95% CI, 23.5%-24.7%]) but increased risk of catastrophic health expenditures (expenses alone: 12.4% [95% CI, 11.2%-13.6%] vs 6.3% [95% CI, 6.2%-6.5%]; including premiums: 26.6% [95% CI, 25.0%-28.1%] vs 16.5% [95% CI, 16.1%-16.8%]; P < .001) relative to the population without cancer. Patients with cancer from low-income families and with full-year private coverage were at particularly high risk of catastrophic health expenditures (including premiums: 81.7% [95% CI, 74.6%-88.9%]). After adjustment, low income was the factor most strongly associated with both insurance churn and catastrophic spending, associated with annual risk increases of 6.5% (95% CI, 4.2%-8.8%) for insurance loss, 17.3% (95% CI, 13.4%-21.2%) for any uninsurance, and 37.4% (95% CI, 33.3%-41.6%) for catastrophic expenditures excluding premiums (P < .001). In adjusted models relative to 2005-2009, full ACA implementation (2014)(2015)(2016)(2017)(2018) was associated with a decreased annual risk of any (continued) Key Points Question Was the Patient Protection and Affordable Care Act (ACA) associated with reductions in risks of insurance churn (ie, gain, loss, or change in coverage) and catastrophic health expenditures for n...