Aug. 20 (Bloomberg) -- Actors posing as drunken sports fans successfully bought alcohol from U.S. stadium vendors almost nine out of 10 tries, according to a University of Minnesota study.

College students who appeared younger than 21, the legal U.S. drinking age, also bought alcohol up to 30 percent of the time without being asked for proof of their age, said the study of 16 sports stadiums and arenas that are home to professional football, basketball, baseball and hockey teams across the U.S. The study is published in the next issue of the journal Alcohol: Clinical & Experimental Research.

Alcohol has been blamed for fueling sports fans' rowdiness, and the National Football League two weeks ago banned spectator drunkenness and unruly behavior. The study shows stadiums need to do more to prevent alcohol sales to drunken and underage fans, said the study's author, Traci Toomey, associate professor at the university's school of public health in Minneapolis.

``Sporting events can be fertile ground for illegal alcohol sales, and the seats provide an especially high-risk environment at events that attract an average of 130 million people each year,'' Toomey said in a telephone interview.

The study's authors sent four actors pretending to be drunk to sporting events in five states. The actors were served more alcohol by vendors walking through stands 89 percent of the time, the study found. They were served 73 percent of the time at concession stands.

Vendors who sell alcohol in the stands check for proof of age and sobriety less vigorously than concession-stand workers because they're under pressure to work faster to avoid blocking other fans' view of the game and are less likely to be monitored by managers, Toomey said.

Youth, Enforcement

The researchers also sent five women and two men who look younger than 21 to sporting events. All were 22 or older. They successfully bought alcohol from roving vendors 30 percent of the time and at concession stands 18 percent of the time, the study found. The stadiums or arenas weren't identified.

``Enforcement is really key,'' Toomey said. ``Enforcement by law enforcement, also enforcement by stadium managers to make sure that the servers get the training they need, but also that they know there's consequences for breaking the law and not following the policies of that stadium.''

To contact the reporter on this story: Gene Laverty in Charlotte, North Carolina, at glaverty@bloomberg.net.

Last Updated: August 20, 2008 16:00 EDT