The Supreme Court on Friday upheld a provision of the Affordable Care Act that requires insurance companies to offer some kinds of preventive care for free.

In a 6-to-3 decision, written by Justice Brett M. Kavanaugh, the majority ruled that a federal task force that determines which preventive health measures insurance companies must cover at no cost to the insured was constitutional.

The decision appears to safeguard coverage for tens of millions of Americans who receive some free health care services, including cancer and diabetes screenings, medications to reduce heart disease and strokes and eye ointment for newborns to prevent infections causing blindness.

Chief Justice John G. Roberts Jr. and Justice Amy Coney Barrett, as well as the liberal wing, Justices Sonia Sotomayor, Elena Kagan and Ketanji Brown Jackson, joined the majority opinion.

The remaining conservative justices, Clarence Thomas, Samuel A. Alito Jr. and Neil M. Gorsuch, dissented.

Mitchell Warren, the executive director of an H.I.V. prevention organization, AVAC, praised the decision, calling its outcome a relief.