The five largest US commercial health insurance companies together enroll 125 million members, or 43 percent of the country’s insured population. Over the past decade these insurers have become increasingly dependent for growth and profitability on public programs, according to an analysis of corporate reports. In 2016 Medicare and Medicaid accounted for nearly 60 percent of the companies’ health care revenues and 20 percent of their comprehensive plan membership. Although headlines have focused on losses in the state Marketplaces created by the Affordable Care Act (ACA), the Marketplaces represent only a small fraction of insurers’ members. Overall, the five largest insurers have remained profitable since passage of the ACA as a result of profits in other market segments. Notably, companies with significant Medicare or Medicaid enrollment have continued to insure beneficiaries in states where the insurers do not participate in Marketplaces. Given the insurers’ dependence on public programs, there is potential to improve access if federal or state governments, or both, required insurers that participate in Medicare or Medicaid to also participate in the Marketplaces in the same geographic area. Such requirements could ensure more viable and less volatile insurance, benefiting people insured within each market as well as those who cycle on and off public and private insurance.