BACKGROUND Conservative outpatient therapy for cervical intraepithelial neoplasia (CIN) by ablative or excisional techniques is widely used. The main objective of this treatment is the prevention of invasive cervical cancer. We assessed the rate of invasive disease and the duration of the risk of developing invasive cervical cancer after such treatment. METHODS Four UK centres have used life-table methods to analyse the long-term results of conservative treatment of CIN. We combined and updated data from these studies to investigate the rate of invasive disease after treatment and the duration of that risk. RESULTS The data comprised 44 699 woman-years of follow-up, with 2116 women under observation 8 years after treatment. 33 women developed invasive cancer, 14 of whom had microinvasion. The cumulative rate of invasion 8 years after treatment was 5.8 per 1000 women and the rate of invasive cancer during this period was 85 (95% CI 60-119) per 100,000 woman-years. The risk of developing cancer did not change throughout the 8 years of follow-up. CONCLUSIONS These data show that conservative outpatient therapy in women with CIN reduces the risk of invasive cancer of the cervix by 95% during the first 8 years after treatment. However, even with careful, long-term follow-up, the risk of invasive cervical cancer among these women is about five times greater than that among the general population of women throughout that period. Careful follow-up is essential for at least 10 years after conservative treatment of CIN.