A nested case-control study was undertaken in the Maribo County cohort of 27,811 women with negative Pap smears. Sixty women who later developed invasive cervical cancer constituted the cases, and five matched controls were selected from the cohort for each case. A total of 633 previous, negative smears for the cases and controls were reviewed independently by two pathologists. The review showed misclassification to be frequent in these smears collected in the period 1966-82. Thirty-five smears were considered positive at the review. The misclassification was differential in respect to the women's later disease status. The odds ratio for patients compared with controls for having at least one positive smear was 22.12 (95% CI 7.54-64.94). We were unable to identify specific characteristics of misclassified smears coming from later cases. Koilocytosis/dyskeratosis, herpes virus changed cells and hyperkeratosis were equally rare in smears from patients and controls. The Maribo County data indicate that the fraction of preventable cases of invasive cervical cancer in women aged 30-64 within the first 5 years after a negative smear could be increased from 62-72% to 83-86%, if misclassification of true positive smears could be eliminated. As a rough estimate, this would be at the cost of a 2% increase in the work load. It should be remembered that there is a large element of extrapolation in applying these results based on relatively poor quality specimens from 1966-82 as compared to a modern screening service.