BACKGROUND: In organized screening programmes for cervical cancer, pre-cancerous lesions are detected by cervical smears. However, during follow-up after a positive smear these pre-cancerous lesions are not always found. The purpose of the study is to analyse positive diagnostic values of smears of at least mild dysplasia, made under the organized screening programmes in the Rotterdam area (1979-1991), and detection ratios of histologically confirmed CIN > or =3, among women participating in these screening programmes. METHODS: Positive diagnostic values and histological detection ratios, by age and history of previous smears, recorded during the national screening programme (1989-1991), were compared with those of the experimental cervical cancer screening project (1976-1984). RESULTS: The positive diagnostic value of a smear with at least severe dysplasia (histologically confirmed CIN > or =3) remains approximately 78%. For smears with mild and moderate dysplasia only lower limits of the diagnostic value could be determined. This was 9% for a smear with mild dysplasia obtained during the national screening programme and 25% and 35% for smears with moderate dysplasia taken during the experimental and national screening programmes respectively. Histological detection ratios for CIN > or =3 in the three rounds of the experimental screening project were 4.7, 2.9 and 1.9. In the first round of the national screening programme the ratio was 4.7, and about three times higher in younger compared to older women. CONCLUSION: Immediate referral for colposcopy after a smear showing moderate dysplasia seems questionable. Whether the increased detection ratio among young women indicates a rise in the risk of cervical cancer is unclear.