The federal legislation requires health insurance plans to allow young adults up to age 26 to remain on their parents' plans and this took effect for insurance plan renewals beginning Sept. 23, 2010.
This provision of the law was designed to address the fact that young adults have traditionally been the age group least likely to have health insurance.
The study published in the Journal of the American Medical Association found of the 2.5 million young adults, whites benefited the most with 1,184,000 being insured under their parents' plan, 736,000 were Latino, 410,000 black, 97,000 Asian, 29,000 American Indian and 42,000 other.
"As a result of the Affordable Care Act, we are making strides in giving every American regardless of race or ethnicity a fair shot at quality, affordable health coverage," Kathleen Sebelius, secretary of the U.S. Department of Health and Human Services, said in a statement. "Because of the law, more and more young adults can breathe a little easier knowing they have health coverage."