To examine the correlations between ras oncogene expression and the development of cervical cancer, the authors studied the reactivity of cervical intraepithelial neoplasia (CIN) and microinvasive lesions of the human uterine cervix by using anti-ras p21 mouse monoclonal antibody rp35. The frequency of positive p21 staining increased with increased grades of malignancy from 17.9% in CIN 1 to 28.9% in CIN 2 and 53.9% in CIN 3, whereas in microinvasive carcinoma it was 50.0%. Furthermore, ten cases of lesions that regressed during a 1-year follow-up period were positive for ras p21 in 20% of cases, but 14 cases of lesions that progressed and developed into higher graded lesions during the 2- to 5-year follow-up period had a 50.0% rate of positive p21 staining. It was concluded that ras oncogene product p21 correlates with the early phase of carcinogenesis of squamous cells of the uterine cervix.