Researchers looked at Pap smear results of 620,063 women tested over a seven-year period. Of the women,128,805 had initial Pap results that were normal and second tests between nine months and three years later. Women screened one, two and three years after normal smears had about the same low rate of cellular abnormalities of significance.

Lead researcher George Sawaya, M.D., Assistant Professor of Obstetrics, Gynecology and Reproductive Sciences at the University of California, San Francisco, says he and his colleagues assumed that the longer women wait, the more likely they are to have a significant abnormality. What they found instead was that it made no difference if women came back for a cervical screening one, two or three years after they had a normal Pap smear.

"The bottom line is that when we are making decisions on the clinical level, we need to inform patients better about the true benefits and risks of screening," Dr. Sawaya says. "If some women are advised by their physicians to extend screening intervals beyond annually, these women should realize the risk of having a bad outcome is very, very small.

"In fact, less screening of low-risk women may be better since the chance of false-positive testing in these women may be much higher than the chance of having significant cervical disease. We often do not inform patients of these benefits and risks since they have not been well-quantified in the past."

American Cancer Society (ACS) guidelines advise women to begin annual screening at age 18 or when they become sexually active but note that they can be screened less frequently after they have three successive normal Pap smears, says Robert Smith, PhD, Director of Cancer Screening.

"We have known for many years that cervical cancer is a very slow-growing disease, and if women have had normal smears, then the probability of an abnormal smear within the next year or two is very low," Dr. Smith says. "Three normal smears is not only a measure of your overall cervical health, but a measure in terms of knowing that the Pap smear is not perfect. And if you miss something in that first year, then you?ll almost certainly catch it in the second. And, while the odds are extraordinarily low that you?ll miss something two years in a row, you could conceivably catch it on the third."

Although these results suggest that some women can have Pap tests less often than yearly, most should continue to get annual checkups. Annual physical exams done at the same time as the Pap test can detect several types of cancers as well as other health problems.

For example, the ACS recommends a yearly pelvic exam for women starting at age 40.  Starting at age 40, women should have annual clinical breast exams and mammograms. Women 20 to 39 should have clinical breast exams and pelvic exams every three years. If they have Pap tests more often than every three years, they should also have a pelvic exam at the same time.

The ACS estimates 12,800 new cases of invasive cervical cancer will be diagnosed in the United States this year, and 4,600 women are expected to die from the disease.

The incidence of cervical cancer declined by more than 70 percent in the U.S. over the past 50 years, thanks to the widespread availability of Pap smears, according to Dr. Sawaya and his colleagues. Of the estimated 50 million smears performed annually in the U.S., most are normal.