The effect of starting screening at age 20 in 1988 was assessed by analysing (a) the age-specific incidence and distribution of stage and histology of invasive diseases, and (b) the detection rates of histologic moderate to high-grade intraepithelial neoplasia (CIN 2-3/AIS), and 1st abnormal cytology and repeat low-grade cytology after follow-up observation. Cancer incidence increased significantly at age 25-34 after 1979 due to early stage squamous cell and adenocarcinoma. After an initial increased rate of preinvasive disease, CIN 3 decreased significantly at age 30-34 after 1988, at age 25-29 after 1993, and levelled out after 1998 at age 20-24. The rates of CIN 2 levelled out after 1998. The rates of repeat low-grade smears decreased after observation at age 20-24 by 80%. The study confirms an increasing rate of preinvasive and invasive disease among younger women and indicates the benefit of starting organised screening at 2-3 year intervals soon after age 20.