In order to determine the clinical significance of numerical aberrations on chromosome 17 in uterine cervical and endometrial neoplasias, we investigated 140 cell specimens obtained from the uterine cervix and endometrium using the fluorescence in situ hybridization (FISH) method. These specimens consisted of ten normal cervical epithelium (NCE), 16 cervical intraepithelial neo­plasias (CIN 1), 15 CIN 2, 35 CIN 3, 11 early invasive squamous cell carcinoma of the uterine cervix (early invasive SCC), 11 invasive SCC, 13 normal endometrium (NE), 17 endometrial hyperplasias (EH), and 12 endometrial ademocarcinomas (EA). After Papanicolaou staining on these specimens was decolorized, FISH was performed using chromosome 17 specific repetitive DNA probes. Signals of centromere of chromosome 17 in marked atypical cells were counted using a fluorescence microscope. There was a signi­ficant difference in the rate of cells with one signal on chromosome 17 between CIN 1 (7.1 ± 0.7%) or 2 (7.0 ± 0.5%) and CIN 3 (12.6 ± 0.9%) (p < 0.01), and also between CIN 3 and early invasive SCC (19.6 ± 1.0%) (p < 0.01). The rate of cells with three signals was significantly increased when the uterine cervical lesions were progressive to CIN 3 (p < 0.01). Cells with five or more signals occurred only in early invasive SCC and invasive SCC. There was a significant difference in the rate of cells with three signals between EH (4.8 ± 0.6%) and EA (11.4 ± 2.1 %) (p < 0.05). Cells with five or more signals occurred only in EA. Examination of the numerical aberrations on chromosome 17 in uterine cervical and endometrial neoplasias has been suggested to be useful as an additional method for the differential diagnosis of these lesions.