DNA cytometric measurements were performed with a TV image analysis system on 90 routine cytologic cervical smears from 18 women after automatic restaining of the smears with the Feulgen method. In the cytologic follow-up all the patients revealed borderline lesions (mild to moderate dysplasia) of the epithelium with koilocytosis. Since DNA aneuploidy is widely accepted as a marker for malignancy, an attempt was made to obtain a DNA diagnosis of prospective malignancy in these cases on the basis of the detection of single aneuploid cells. Because koilocytosis strongly suggests human papillomavirus infection, euploid polyploidy might result in up to 8c (c = haploid amount of DNA). Therefore, the aneuploidy detection threshold was set to 9c, excluding values around 16c and 32c (+/- 12.5%) to avoid false-positive diagnoses in cases of higher polyploidy values. The DNA diagnosis of malignancy was made in 13 patients. In 12 of them, carcinoma in situ was diagnosed in the histologic follow-up, resulting in 100% sensitivity of the DNA diagnosis. In five patients no cells greater than 9c were measured, leading to a benign DNA diagnosis. In the histologic follow-up, six patients revealed normal epithelium or moderate dysplasia. Thus, the specificity of the DNA diagnosis was 83.3%.