* Health reform gave young adults coverage under parents

* 6.6 million young adults newly under parents' plans

* But young still struggle with medical bills, debt

WASHINGTON, June 8 (Reuters) - U.S. healthcare reform likely enabled about 6.6 million young adults to join their parents' health insurance plans last year, a report found on Friday, though problems with medical bills and debt remained an issue.

President Barack Obama's 2010 healthcare reform law allowed young adults - who previously had the nation's highest uninsured rate - to stay on their parents' private insurance plans through age 26.

This provision is perhaps the single most popular element of the Affordable Care Act, the nation's most sweeping healthcare legislation in nearly 50 years and Obama's signature domestic policy achievement.

Polls show Americans are sharply divided about the law ahead of a Supreme Court ruling on its constitutionality by the end of June.

The Commonwealth Fund, a nonprofit organization that analyzes healthcare issues, polled 1,863 adults between the ages of 19 to 25 and found 47 percent of them joined or remained on their parents' plans between November 2010 and November 2011.

This would translate into about 13.7 million young adults in the broader population.

Of those, 6.6 million would likely not have been able to be on their parents' plans before the law's passage, as they were not enrolled in college full time or had already graduated. Most insurance plans already allow full-time college students to stay on their parents' plans.

The results compared to a U.S. government survey that last year found about 21.6 million young adults had private health insurance - either through their parents, their jobs or other means - which was 2.5 million higher than before the law was passed.

But the Commonwealth Fund also found 36 percent of young adults between the ages of 19 and 29 - a slightly bigger group - had trouble paying medical bills or said they were paying off medical debt. And among those without insurance, this group rose to 51 percent.

Sara Collins, one of the study's authors and vice president at the Commonwealth Fund, said some young people need maternity coverage, which is often expensive but may not be provided by insurance plans.

Young adults also have the highest rate of injury-related visits to the emergency room - even above children and the elderly - and may have other health conditions such as HIV or the human papillomavirus (HPV).

The survey, conducted online, has an average sampling error margin of 3 percentage points. (Reporting by Anna Yukhananov; Editing by Lisa Shumaker)