Nucleolar organizer regions (NORs) in 70 cases of precancerous and cancerous lesions of the uterine cervix were investigated. The greater the degree of dysplasia, the higher the mean number of silver-stained NOR (Ag-NOR) dots (mild dysplasia, 2.2; moderate dysplasia, 2.5; severe dysplasia, 3.1). The mean number of dots in moderate or severe dysplasia was significantly higher than that in normal squamous epithelial cells. The mean numbers of dots in carcinoma in situ (CIS) and invasive carcinoma were 3.8 and 3.7, respectively, and were significantly higher than those in normal squamous epithelial cells, mild dysplasia, moderate dysplasia, and severe dysplasia. The mean number of Ag-NOR dots in the cases of mild dysplasia that progressed to CIS was 2.7, which was significantly higher than that in the cases that regressed. In precancerous lesions, the correlation coefficient between the mean number of Ag-NOR dots and the mitotic index was 0.552 (p less than 0.01), whereas no correlation was noted between these measurements in CIS and invasive carcinoma. In conclusion, the number of Ag-NOR dots is a good marker to detect cell proliferation in precancerous lesions and their malignant tendency.