Objective The objective of this study was to evaluate the impact of cervical cytology screening on the prevalence of cervical cytological results in women, as a function of age and test interval. Methods This is a cross-sectional study of cytology screening data of 2,002,472 tests obtained from previously screened women and 217,826 tests from unscreened women. The central cytopathology laboratory database was analyzed. The tests were collected for screening purposes from Campinas metropolitan region, Brazil. A prevalence ratio (PR) with a 95% confidence interval was calculated for the screened women, in relation to the unscreened women, and for different tests intervals. Protection afforded by screening (1-PR) was calculated. Results For high-grade squamous intraepithelial lesion, the PR was 0.97 (0.83-1.13) for women aged 20 years or younger and 0.99 (0.86-1.14) for women aged 20 to 24 years, decreasing significantly in women aged 25 to 29 years (PR, 0.63 [0.52-0.76]). The PR for squamous cell carcinoma, adenocarcinoma in situ (AIS), and invasive adenocarcinoma showed a significant reduction in all age groups older than 30 years. For the age group ranging from 30 to 59 years, protection for squamous cell carcinoma, AIS, and invasive adenocarcinoma was 83% or higher for screening intervals from 1 to 5 years. Protective effect was not demonstrated for screening intervals longer than 5 years for AIS and invasive adenocarcinoma. Conclusions Cytology screening is effective at preventing cytological high-grade squamous intraepithelial lesion, squamous cell carcinoma, AIS, and invasive adenocarcinoma. On the basis of cytological results, protection against AIS and invasive adenocarcinoma was observed with screening intervals shorter than 5 years. Cytological screening in women 25 years or younger should be critically evaluated.242321328