WASHINGTON — Lonnie Bowen Jr. was once a social worker, but for 17 years the Vietnam War veteran has slept on the streets off and on as he's battled substance abuse and mental health problems.

"It's been a hard struggle," said Bowen, 62, as he rolled a cigarette outside a homeless processing center in downtown Philadelphia, where he planned to seek help for his drug and alcohol problem, as he has before.

Every night, hundreds of thousands of veterans like Bowen are without a home.

Veterans make up more than one in four homeless people in the United States, though they are only 11 percent of the general adult population, according to a report to be released today by the National Alliance to End Homelessness, a public education nonprofit group.

And homelessness is not just a problem among middle-age and elderly veterans. Younger veterans from Iraq and Afghanistan are trickling into shelters and soup kitchens seeking services, treatment or help with finding a job.

The Veterans Affairs Department has identified 1,500 homeless veterans from the current wars and says 400 of them have participated in its programs specifically targeting homelessness.

The National Alliance to End Homelessness based the findings of its report on numbers from Veterans Affairs and the Census Bureau. Data from 2005 estimated that 194,254 homeless people out of 744,313 on any given night were veterans.

In comparison, the VA says that 20 years ago, the estimated number of veterans who were homeless on any given night was 250,000.

Some advocates say the early presence of veterans from Iraq and Afghanistan at shelters does not bode well for the future. It took roughly a decade for the lives of Vietnam veterans to unravel to the point that they started showing up among the homeless. Advocates worry that intense and repeated deployments leave newer veterans particularly vulnerable.

"We're going to be having a tsunami of them eventually because the mental health toll from this war is enormous," said Daniel Tooth, director of veterans affairs for Lancaster County, Pa.

While services for homeless veterans have improved in the past 20 years, advocates say more financial resources still are needed. With the spotlight on the plight of Iraq veterans, they hope more will be done to prevent homelessness and provide affordable housing to the younger veterans while there's a window of opportunity.

"When the Vietnam War ended, that was part of the problem. The war was over, it was off TV, nobody wanted to hear about it," said John Keaveney, a Vietnam veteran and a founder of New Directions in Los Angeles, which gives veterans help with substance abuse, job training and shelter.