BACKGROUND: Trends in CIN 2-3+ lesions affect the choice of target age group and screening interval. METHODS: The following analyses were performed for CIN 2-3+ following 1st to 5th screening visits in the 20-34/35-69 age groups during 1979-2002: a) the relative risk of age and calendar year; b) the cumulative incidence rate after normal or low or higher grade cytology; c) the proper screening interval to detect these lesions at a fixed risk level after normal screening visits; d) the predictive power of these lesions for diagnosis of cancer after first biopsy; e) the prevalence rates in the 20-24/25-29 year age groups at first and at all subsequent screening visits. RESULTS: The cumulative rate decreased with the number of screening visits and advancing age. After normal and low-grade cytology the cumulative rate increased linearly but slowly with time but after higher grade cytology the rate was relatively stable within one year. Age and calendar year are important risk factors in younger women. The prevalence of CIN 2-3+ increased significantly at 1st visit in the 20-24 age group at the same time as the population prevalence in the 25-29 age group decreased significantly. CIN 2 was at lower risk than CIN 3 of being diagnosed with cancer. CONCLUSIONS: The results indicate that screening should preferably start before age 25 with a maximum interval of 3 years, whereas the interval can be extended to 4-5 years at age 35-40. CIN 2 have a different risk profile compared to CIN 3.