All women in Nijmegen, The Netherlands, with a histological diagnosis of severe dysplasia, or carcinoma in situ, or invasive carcinoma were investigated to see whether they had participated in a population screening programme. Within two years of diagnosis of a negative cervical smear, 45 women were found to have histologically confirmed severe epithelial abnormality of the cervix. From the same population as these apparently false negative cases, the number of true positive cases was available, and hence the sensitivity of cervical screening for severe dysplasia, or carcinoma in situ, and invasive carcinoma could be assessed. This was found to be 83% after two years. The laboratory procedures which led to the high sensitivity for the cervical cytodiagnosis were analysed. Experienced sample takers and cytotechnologists are very important and can reduce sample and screening errors. A good administrative system is necessary to guarantee proper follow up for women with abnormal findings in their cervical smears.