Proliferative activity of various human uterine cervical lesions obtained by conization was examined by measuring the proportion of mitotic cells including abnormal mitosis and the nuclear DNA content by a microspectrophotometer. Twelve ratios of mitotic cells calculated in the present study showed a step-wise increase from normal squamous cell epithelium through various grades of dysplasia to carcinoma in situ (CIS). The great difference between moderate and severe dysplasia was observed. All ratios in CIS with bulky outgrowth (CIS(b)) were the highest. The nuclear DNA content in various lesions also indicated the great difference between moderate and severe dysplasias in the DNA histograms. Severe dysplasia had a wider distributed DNA histogram without distinct modes similar to those in CIS and the non-invasive areas of the microinvasive carcinoma. These results may suggest that severe dysplasia but not slight or moderate dysplasia is a direct precursor lesion for uterine cervical epidermoid carcinoma.