The cell cycle is the set of events that is responsible for the duplication of the cells. Recent studies indicate that cell cycle regulatory proteins, mainly the cyclins and cyclin-related genes, can be critical targets during oncogenesis. The genes and gene products normally control specific events in the cell cycles, particularly during the late G1 and early S phase and G2/M phase. A large body of date implicates cyclins in oncogenesis. The first evidence came from human cyclin A in oncogenesis. Cyclin A is expressed from the late G1 phase through the M-phase of the cell cycle. Cyclin A is known as positive regulator of cell cycle and participates in the tumorigenesis. Overexpression of cyclin A has been reported in several cancers. Ki-67 is a nuclear protein expressed during the cell cycle except in Go. The labeling index of Ki-67 in the tumor cell nuclei has been used as a good prognostic factor. In this study, we compared labeling index of cyclin A and Ki-67 to assess the feasibility between them with 30 cases of cervical intraepithelial neoplasia(CIN) and 20 cases of invasive squamous cell carcinoma(SCC)by immunohistochemistry. The results were as follow; 1. Cyclin A expressed in normal parabasal cells and their labeling index was 0.8+/-0.4%, while in CIN and invasive SCC 65.5+/-9.4% and 86.5+/-12.3% respectively. Ki-67 expressed in normal parabasal cells as 1.3+/-0.7% while in CIN and invasive SCC as 77.8+/-12.9% and 92.2+/-17.6% respectively. 2. In CIN, the expression of cyclin A increased according to the grades of the CIN as 32.5+/-5.7%, 75.8+/-9.0%, and 83.2+/-13.4% in CIN I, II and III respectively. The expression of the Ki-67 also increased according to the grades of the CIN as 51.8+/-9.8%, 87.9+/-11.3%, and 93.6+/-17.5% respectively in CIN I, II and III. 3. There was no differences of cyclin A and Ki-67 expressions according to the histologic types of invasive SCC. Above results suggests that the cyclin A labeling index could be used as a marker of tumor progression in the uterine cervical carcinoma as Ki-67.