Conization specimens with a histologic diagnosis of cervical intraepithelial neoplasia III were reviewed with respect to cytology findings in the three months preceding conization. In 29% of the cases, one or more false-negative cytology reports preceded conization. No dysplastic cells were found on review of the false-negative cytology slides from these cases. Morphometric analysis of surface dysplasia in the conization specimens was performed to explain these findings. A statistically significantly greater surface area of dysplasia and greater spread of dysplastic cells in the endocervical canal was found in the positive cytology when compared to the false-negative cytology cases. No difference in vaginal spread of dysplasia existed between the two groups. The endocervical canal was sampled with a moist cotton swab in all of the cases. The results suggest that small numbers of dysplastic cells high in the endocervical canal may not be effectively sampled by cotton swabs in cases of false-negative cervical cytology, lowering screening sensitivity.