Abnormal expression of the spindle assembly checkpoint proteins causes tumor cell aneuploidy, which has been reported in various malignancies. The expression of mitotic arrest deficient 2 (MAD2) and cell-division cycle 20 homolog (CDC20), the key spindle assembly checkpoint proteins, has not been studied in cervical carcinogenesis. In this study, we compared the expression of MAD2 and CDC20 in 332 cases, including normal cervical tissues, low-grade squamous intraepithelial lesions, high-grade squamous intraepithelial lesions (HSILs), and invasive squamous cell carcinomas (SCCs). Both MAD2 and CDC20 were overexpressed in the nuclei or cytoplasm of dysplastic and malignant tumor cells. The frequency of MAD2 overexpression was markedly increased from undetectable (0/100) in normal cervical tissues and 2% (1/50) in low-grade squamous intraepithelial lesions to 67.1% (53/79) in HSILs and 52.4% (54/103) in SCCs. Similarly, CDC20 was overexpressed in 49.4% (39/79) of HSILs and 22.3% (23/103) of SCCs, whereas CDC20 was not detectable (0/100) in normal cervical tissues and overexpressed only in 8.0% (4/50) of low-grade squamous intraepithelial lesions. In SCC cases, MAD2 overexpression correlated with a patient age of less than 60 yr (P=0.043), nonkeratinizing histologic type (P=0.018), and a lesser degree of stromal invasion (P=0.026). In conclusion, MAD2 and CDC20 overexpression was increased in HSILs and SCCs, suggesting their involvement in the initiation of cervical cancers. Controlling CDC20 and MAD2 expression may be a therapeutic strategy for cervical cancer.