Researchers at the Bradley Hasbro Children's Research Center in Providence said these findings mirror recent data that show anal sex rates among adults doubled from 1995 to 2004.

"The topic of anal intercourse is often considered taboo -- especially when discussed in the context of youth relationships -- even though we know that this behavior is a significant risk factor for HIV and other sexually transmitted infections," lead author Celia Lescano said in a statement.

Researchers assessed the sexual behavior of 1,348 at-risk teens and young adults between the ages of 15 and 21 who had unprotected sex in the previous three months. They found that 16 percent had engaged in heterosexual anal intercourse within the time frame, with condoms being used 29 percent of the time.

Females who had heterosexual anal sex were more likely to be living with their partners, to have two or more sexual partners and to have previously experienced coerced intercourse, the study said.

The study, published in the American Journal of Public Health, said males who engaged in heterosexual anal intercourse were more likely to identify themselves as being homosexual, bisexual or undecided.

Ninety-two percent of the participants defined themselves as heterosexual, 56 percent were female, about half were African-American, 24 percent Hispanic and 20 percent white, the researchers said.