WASHINGTON -- Tens of thousands of U.S., Mexican and Canadian children and youths become victims of juvenile pornography, prostitution and trafficking each year. So significant is the problem that even most law-enforcement and child-welfare officials do not realize its scope.

"Child sexual exploitation is the most hidden form of child abuse in the U.S. and North America today. It is the nation least recognized epidemic ," said Richard J. Estes, a University of Pennsylvania professor of social work and the author of "The Commercial Sexual Exploitation of Children in the U.S., Canada and Mexico." Neil Weiner of Penn Center for the Study of Youth Policy co-authored the international report.

The three-year project was funded by the National Institute of Justice of the U.S. Department of Justice, the W.T. Grant Foundation, the Fund For Nonviolence and the Research Foundation of the University of Pennsylvania.

Estes reported that his and Weiner research identified 17 groups of children in the U.S. who are at "substantial risk" of being sexually exploited.

"The largest of these groups are runaway, thrownaway and other homeless American children who use urvival sexto acquire food, shelter, clothing and other things needed to survive on America streets," Estes said.

"These children are solicited for sex repeatedly by men, many of whom are married and have children of their own," Estes said. "Like other groups of sexually exploited persons, street children are exposed to violence, drug abuse, rape and, sometimes, even murder at the hands of the pimps, ustomersand traffickers that make up their world."

Estes also reported that some U.S. children engage in commercial sex while living at home.

"The majority of these children trade sex for money or for more expensive clothes and other consumer goods. Most of the ustomersof these children are members of their own junior and senior high school peer groups," he said.

Many of these children live in secure middle-class homes, and few parents are aware of their children involvement in pornography or prostitution.

This group also includes American youths who cross into Canada or Mexico in pursuit of cheaper drugs, alcohol and sex. Mexican authorities report that border towns are little more than "cantinas for America youth," Estes said.

The sexual exploitation of children is not limited to particular racial, ethnic or socioeconomic groups, according to the Penn professorsreport, although children from poorer families appear to be at a somewhat higher risk of commercial sexual exploitation. In fact, most of the street children encountered in the study were Caucasian youths who had run away from middle-class homes.

But, "a disproportionate number of street youth have histories of recurrent physical or sexual abuse at home and took to the streets in a desperate effort to bring their abuse to an end," Estes said. "It is ironic that running away from home increases their risk of physical violence and sexual abuse."

Many street youths use drugs "to deal with the emotional pain of being sexually victimized at home and, once on the streets, by four to 10 ustomersa day," Estes said.

Just as the exploited children come from all parts of society, so do the perpetrators of sex crimes against children. These sexual predators include relatives and other adults known and trusted by the children or their families.

"Despite popular notions to the contrary," Estes said, "strangers commit fewer than 4 percent of all the sexual assaults against children."

In the case of street children, their "customers" include pedophiles, pederasts, pimps and traffickers.

Other customers are transient males, including members of the military, long-haul truck drivers, seasonal workers, conventioneers and sex tourists.

"In the U.S., child sexual exploitation affects as many boys as girls, but boys are less well served by human-service and law-enforcement systems because of the widespread belief that boys are better able than are girls to fend for themselves," Estes reported.

Given the high levels of emotional dysfunction, drug abuse and violence that exists for boys living on America streets, however, this is not true. In time, many boys shift from being victims of sexual abuse to victimizing other boys and girls as pimps and traffickers.

Other groups of commercially sexually exploited children in the U.S. include girls in gangs; transgender street youths; foreign children brought into the U.S. illegally, especially from Africa, Asia, Eastern Europe and elsewhere in the Americas; and U.S. youth who are trafficked nationally and internationally as part of organized sex crime rings.

Estes and Weiner have identified an 11-point action agenda focused on eliminating the further commercial sexual exploitation of America youth.

"There is an urgent need," Estes said, "for systematic public and professional education on the causes, nature and extent of child sexual exploitation in the United States. The situation in the U.S. must be understood within the broader content of child sexual exploitation occurring throughout both the North American region and the rest of the world. Only through such understanding will the U.S. be able to act decisively in protecting her children from such heinous abuse."

He also called for earlier identification and more intensive supervision of sexually offending adults and juveniles as urgent priorities in protecting children from sexual exploitation.