CHICAGO — At least one in four teenage American girls has a sexually transmitted disease, suggests a first-of-its-kind federal study that startled some adolescent health experts.

Some doctors said the numbers might be a reflection of both abstinence-only sex education and teenagers’ own sense of invulnerabilty. Because some sexually transmitted infections can cause infertility and cancer, American health officials called for better screening, vaccination, and prevention. Only about half of the girls in the study acknowledged having sex. Some teenagers define sex as only intercourse, yet other types of intimate behavior, including oral sex, can spread some diseases.

Among those who admitted having sex, the rate was even more disturbing — 40% had an STD.

“This is pretty shocking,” an adolescent medicine specialist at Montefiore Medical Center’s Children’s Hospital in New York, Dr. Elizabeth Alderman, said.

“To talk about abstinence is not a bad thing,” but teenage girls — and boys too — need to be informed about how to protect themselves if they do have sex, Dr. Alderman said.

The overall STD rate among the 838 girls in the study was 26%, which translates to more than 3 million girls nationwide, researchers with the U.S. Centers for Disease Control and Prevention found. They released the results yesterday at an STD prevention conference in Chicago.

“Those numbers are certainly alarming,” sex education expert Nora Gelperin, who works with a teenager-written Web site called sexetc.org, said. She said they reflect “the sad state of sex education in our country.”

“Sexuality is still a very taboo subject in our society,” she said. “Teens tell us that they can’t make decisions in the dark and that adults aren’t properly preparing them to make responsible decisions.”

The president of Planned Parenthood Federation of America, Cecile Richards, said the study shows that “the national policy of promoting abstinence-only programs is a $1.5 billion failure, and teenage girls are paying the real price.”

Similar claims were made last year when the government announced the teenage birth rate rose between 2005 and 2006, the first increase in 15 years.

The new study by CDC researcher Dr. Sara Forhan relied on slightly older data. It is an analysis of nationally representative records on girls ages 14 to 19 who participated in a 2003-2004 government health survey.

The teens were tested for four infections: human papillomavirus, or HPV, which can cause cervical cancer and affected 18% of girls studied; chlamydia, which affected 4%; trichomoniasis, 2.5%; and genital herpes, 2%.

The director of the CDC’s division of STD prevention, Dr. John Douglas, said the results are the first to examine the combined national prevalence of common sexually transmitted diseases among adolescent girls. He said the data, now a few years old, likely reflect current prevalence rates.