The sensitivity and specificity of cervical cytology and of cervicography in the detection of CIN and invasive cancer of the cervix were determined in a screening programme of an asymptomatic population of 418 women. Because all of the subjects were examined colposcopically and biopsies were taken from any suspicious areas, the presence or absence of disease was determined by the histology of directed cervical biopsies independently of the screening methods being tested. Cervicography correctly identified 24 of the 27 women with CIN or invasive cancer, whereas cytology detected only 14 (sensitivity of 0.89 vs. 0.52;P< 0.01). Cervicography detected CIN in 11 women with negative smears, six having CIN III. Only one case of CIN I was detected by cytology when cervicography was negative. Two cases of CIN II were not detected by either method. Inclusion in the analysis of the results from 23 women with technically defective cervigrams reduced the size of the difference in sensitivity, but it remained significant (0.74 vs. 0.48;P< 0.05). Cervicography was only slightly less specific than cytology (0.92 vs. 0.94; NS). Cervicography appears to be more effective than cytology in screening for cervical premalignancy but may be more demanding of those who perform the test.