BACKGROUND: Although contemporary guidelines suggest that the intervals between Papanicolaou tests can be extended to three years among low-risk women with previous negative tests, the excess risk of cervical cancer associated with less frequent than annual screening is uncertain. METHODS: We determined the prevalence of biopsy-proven cervical neoplasia among 938,576 women younger than 65 years of age, stratified according to the number of previous consecutive negative Papanicolaou tests. Using a Markov model that estimates the rate at which dysplasia will progress to cancer, we estimated the risk of cancer within three years after one or more negative Papanicolaou tests, as well as the number of additional Papanicolaou tests and colposcopic examinations that would be required to avert one case of cancer given a particular interval between screenings. RESULTS: Among 31,728 women 30 to 64 years of age who had had three or more consecutive negative tests, the prevalence of biopsy-proven cervical intraepithelial neoplasia of grade 2 was 0.028 percent and the prevalence of grade 3 neoplasia was 0.019 percent; none of the women had invasive cervical cancer. According to our model, the estimated risk of cancer with annual Papanicolaou tests for three years was 2 in 100,000 among women 30 to 44 years of age, 1 in 100,000 among women 45 to 59 years of age, and 1 in 100,000 among women 60 to 64 years of age; these risks would be 5 in 100,000, 2 in 100,000, and 1 in 100,000, respectively, if screening were performed once three years after the last negative test. To avert one additional case of cancer by screening 100,000 women annually for three years rather than once three years after the last negative test, an average of 69,665 additional Papanicolaou tests and 3861 colposcopic examinations would be needed in women 30 to 44 years of age and an average of 209,324 additional Papanicolaou tests and 11,502 colposcopic examinations in women 45 to 59 years of age. CONCLUSIONS: As compared with annual screening for three years, screening performed once three years after the last negative test in women 30 to 64 years of age who have had three or more consecutive negative Papanicolaou tests is associated with an average excess risk of cervical cancer of approximately 3 in 100,000.