Anthem Inc. said it will pull out of the Affordable Care Act health-insurance exchange in Ohio next year, a move that likely will leave 20 counties in the state with no available ACA marketplace plans.

The move is the first withdrawal by Anthem ANTM, +2.60% , a huge exchange insurer that has warned repeatedly that it was weighing its future in the ACA marketplaces, which sell health insurance to individuals. The decision will likely create alarm in the other 13 states where Anthem offers individual health-insurance plans. So far, the insurer has filed 2018 ACA plans with regulators in other states, including Virginia, Maine and Connecticut, but it could still pull back.