To estimate the risk of being diagnosed with cervical intraepithelial neoplasia (CIN) 2/3 or invasive cervical cancer (ICC) based on diagnostic and screening procedures performed after a diagnosis of atypical squamous cells of undetermined significance (ASCUS) and to compare this risk to that in women with a normal Pap smears. A 7-year, prospective, cohort study was performed in the Norwegian population-based, coordinated screening program. After excluding women in the midst of follow-up of an abnormal Pap smear or with a history of CIN 2/3 or ICC, the study population consisted of women 25-69 years of age with a normal (n = 526,661) or ASCUS Pap smear (n = 10,037) in 1995-1996. Risk estimates were calculated by logistic and parametric survival regression. Within 7 years of an ASCUS smear, 1,017 women (10.1%) were diagnosed with CIN 2/3 and 62 (0.62%) with ICC. Women with an ASCUS index Pap smear had a relative risk of 15-30 of being diagnosed with histologically verified CIN 2/3 or ICC within the first 2 years of follow-up as compared to women with a normal index smear. In long-term follow-up, women with an ASCUS index smear followed by a normal smear, which cancelled further clinical follow-up, were at > 3.5 times higher risk of both CIN 2/3 and invasive cancer as compared to women with a normal index smear. Pap smear follow-up of women with an ASCUS smear does not identify all women at higher risk of CIN 2/3 and ICC. Other diagnostic procedures should be implemented to improve the screening program.