Seattle ? The stubborn epidemic of sexually transmitted diseases among young adults has long been largely blamed on the risky ways of college students. But research out Friday by Group Health shows young, single, working women are having even more unprotected sex — and with more partners — than students.

The study of 1,100 single women — two-thirds from the Puget Sound area and one-third from North Carolina — pinpoints a previously overlooked high-risk group: sexually active women ages 18 to 25 outside the college setting. Among such women, 61 percent reported having sex without a condom in the past three months, compared with 56 percent of female college students.

Study co-author Delia Scholes, associate investigator at Group Health’s Center for Health Studies, said she was alarmed that despite having unsafe sex, these women weren’t worried about the risks. Seventy-eight percent felt they were at low risk for catching an STD.

Contrary to their feelings of immunity, women in this age group are particularly hard hit by STDs. About 70 percent of STDs diagnosed in Washington are in women under 25. Likewise, 77 percent of the state’s 15,000 reported cases of chlamydia last year were in women between ages 15 and 24. The second-most-common STD in Washington, gonorrhea, infects women ages 20 to 24 more than any other age group — with 194 cases per 100,000 people, said Mark Aubin, STD field-services coordinator for the state’s Department of Health.

“The great majority of the consequences of STDs occur in women,” Scholes said. “They can develop pelvic inflammatory disease and infertility” from untreated chlamydia.

The telephone survey was published Friday in the journal Preventive Medicine. Among nonstudents, 67 percent of single women ages 21 to 25 reported having unsafe sex, compared with 53 percent of the younger age group, 18 to 20. And this decrease in condom use wasn’t a reflection of having sex with fewer partners — those with more sex partners were even more likely to have unprotected sex than those in monogamous relationships.

“It’s very counterintuitive,” said Lisa Gilbert, director of research at the American Social Health Assn., an STD-awareness group. “You would certainly expect they would get less wild, more mature and make better decisions as they got older.”

She speculates these issues are more on the minds of college students because they are heavily targeted by safe-sex messages on campus, but for those who don’t go to college, or people who have finished college, no one is there reminding them the dangers still exist.

And while college students are far from paradigms of safe sex, this study suggests the messages are working somewhat because students are still safer than the young women out of college. In addition to having less unprotected sex, fewer had multiple sex partners in the past year — 52 percent of students compared with 62 percent of nonstudents.

“Clearly, if you look at their behaviors, these young working women are right up there in terms of risk,” said Dr. Kimberly Yarnall, the study’s lead author from Duke University. Though the researchers didn’t ask about occupations, because the study population was taken from HMOs, except for 15 percent who had Medicaid, Yarnall said she assumed the rest of the nonstudents had jobs because they had health insurance.

Yarnall said she was stumped about the risky behaviors. “I don’t know if this is part of a trend where people are becoming complacent in terms of protecting themselves,” she said. “Or maybe by this age they’ve gotten desensitized to the issue.”

She said what she could conclude from the study, however, was that primary-care providers need to spend more time with young women screening for risk behaviors and explaining the consequences of unprotected sex.