Cyclin/cdc complexes are known to function in cell-cycle regulation. Cyclin D1/cdk4 and -6 complexes, which functions as a G1-S checkpoint and cyclin B1/cdc2 complexes, a G2-M checkpoint are essential for DNA synthesis and mitosis, respectively. Thus, dysregulated overexpression of cyclins appears to be involved in uncontrollable cell proliferation and early tumor development. We investigated the expression and proliferative index of cyclin D1 (PI_{cyclin D1}), cyclin B1 (PI_{cyclin B1}) and Ki-67 (PI_Ki-67) using immunohistochemical staining on 15 cases of ductal hyperplasia (DH), 26 cases of atypical ductal hyperplasia (ADH) and 43 cases of ductal carcinoma in situ (DCIS) of the breast in order to evaluate whether these cyclins are associated with abnormal cell proliferation and play a role in tumor development from ADH to carcinoma. Furthermore, we investigated whether the expression and proliferative index of the cyclins and Ki-67 are correlated with the histologic grade according to the Van Nuys classification and with the histologic subtype according to traditional classification. Finally, we estimated the correlation coefficient among PI_{cyclin D1}, PI_{cyclin B1}, PI_Ki-67 and estrogen receptor in ADH and DCIS. The expression of cyclin D1 was detected in 39.5% of DCIS and 7.7% of ADH cases. In the DH cases, expression of cyclin D1 was not found. Expression of cyclin B1 was also detected in 69.7% of DCIS, 50.0% of ADH and 93.3% of the DH cases. The PI_{cyclin D1} was significantly different among these three groups. Moreover, the PI_{cyclin D1} and PI_Ki-67 were differed significantly between the low grade DCIS and ADH cases. However, PI_{cyclin B1} only appeared to be significantly different between the total DCIS and ADH. Results of the correlation coefficient among PI_{cyclin D1}, PI_{cyclin B1} and PI_Ki-67 were positively correlated with each other. No significant correlation was found between the expression of ER and cyclin D1 in ADH and DCIS. In summary, our results support the hypothesis that a cyclin D1 and cyclin B1 protein aberration, along with Ki-67, may act as a relatively early event in the tumor development from ADH to carcinoma.