The nuclear DNA content of 43 abnormal squamous lesions from 31 DES-exposed females was studied with Feulgen microspectrophotometry and correlated with histopathologic findings and follow-up information. These lesions included 12 specimens of squamous metaplasia in 12 subjects, 24 of mild dysplasia in 19 subjects, and 7 of moderate to severe dysplasia in 4 subjects. Based on the distribution of nuclear DNA content, mature and immature squamous metaplasias were euploid, having a normal diploid (2N) DNA value. Mild dysplasias were usually polyploid, having multiples (4N, 8N) of normal diploid DNA content. Only one example of mild dysplasia was euploid. All the moderate and severe dysplasias except one were aneuploid, having DNA values other than euploid or polyploid. Follow-up studies by cell samples and biopsies showed that dysplasias having a polyploid or euploid DNA content may rarely persist as such or progress to a more severe form (3 of 20 patients), in contrast to aneuploid dysplasias in which persistence appears to be more common (2 of 3 patients). These results indicate that immature squamous metaplasia and mild dysplasia are frequently transient in DES-exposed females, unlike the more rarely observed moderate and severe dysplasias, which appear to be persistent based on the study of a few cases.