Military gambling is a big business. About $2 billion flows through military-owned slot machines at officers' clubs, activities centers and bowling alleys on U.S. overseas bases each year.

Most flows back out as jackpots, but 6 percent remains with the house, about the same ratio as in Las Vegas.

The armed forces take in more than $120 million a year from on-base slot machines and $7 million from U.S. Army bingo games at home.

The funds help pay for recreational programs.

But even the military has acknowledged that the armed forces are heavily populated by people who may be especially vulnerable to gambling addiction: athletic, risk-taking young people who are experiencing severe stress and and anxiety.

"And wartime is an environment that is probably creating more vulnerability than usual," said Christine Reilly, executive director of the gambling addiction research institute at Cambridge Health Alliance, a teaching affiliate of the Harvard Medical School.

Over four years ago, the U.S. Congress ordered the Pentagon to study how on-base slot machines affected military families. The Pentagon hired PricewaterhouseCoopers to do the study, but it ended the contract after a few months and completed the study itself.

The final report provided no new data about the rates of problem gambling. But it did caution Congress that the military could not maintain many popular programs, like golf courses and family activity centers, "without slot machine revenue or a significant new source of cash."

One consultant who worked with PricewaterhouseCoopers was Rachel Volberg, a medical sociologist who runs Gemini Resources, which measures gambling rates around the world. "We met a great deal of defensiveness, both in Washington and on base," she said. "Everyone was very concerned that those revenues might go away."

Slot machines are "a very profitable operation," said Peter Isaacs, the chief operating officer of the U.S. Army's Community and Family Support Center, which runs the largest military slot machine program.

Isaacs added, "The vast majority of the troops use the machines responsibly."

Most cited slot machines as their primary form of gambling, although five said they spent "significant time playing bingo" as well.

Slot machines have been a fixture of military life for decades. They were banned from domestic bases in 1951 after a series of scandals and removed from U.S. Army and Air Force bases in 1972.

But 1,500 machines remained on Navy and Marine Corps bases overseas and in 1980, the army and Air Force started to restore the machines at many overseas bases.

Today, there are approximately 4,150 modern video slot machines at military bases in nine countries, according to Isaacs and an Air Force spokesman.