Researchers said the study, based on a national survey of 23,000 American teens and young adults from 2002 to 2006, is the first large-scale survey to delve into the questions of where teens get their alcohol and how they socialize when they drink it.
Just three in 10 paid for alcohol themselves the last time they drank, the study concluded.
"When they paid for it, they drank more," says James D. Colliver, PhD, a statistician with the National Survey on Drug Use and Health and a co-author of the study.
Underage drinkers who bought their own alcohol the last time they drank told researchers they drank an average of six drinks, while those who got it for free drank about four drinks. Females were more likely than males to have gotten their drinks for free.
And one in 16 kids who drank in the past month told researchers they got their alcohol from their parents.
"In far too many instances parents directly enable their children's underage drinking," Acting Surgeon General Steven K. Galson, MD, says in a statement.
About 30% of underage Americans report drinking in the past month. While teens' use of many other illegal drugs has dropped, underage drinking has stayed mostly constant since 2002, according to the annual study.
Several factors have been blamed for stubborn drinking rates. A report issued earlier this week from the Center for Alcohol Marketing and Youth at Georgetown University cited large increases in cable TV advertising by alcoholic beverage companies.
Thursday's study also found young people were more likely to drink in someone else's home than in their own house.
"They tended to have more drinks if they were at someone else's home," Colliver told WebMD.
Despite the new data on where teens obtain and drink alcohol, parents are only half the time aware their kids are drinking, according to another study published in April by the Substance Abuse and Mental Health Administration.
That study showed that 54% of mothers and 47% of fathers of kids who drink knew their children drank alcohol.