PURPOSE: Cyclins are groups of proteins that play a role as a major regulator of the G1 restriction point promoting inactivation of the retinoblastoma protein. The cyclin D1 gene, CCND1, is amplified in approximately 20% of breast carcinomas and the protein is reportedly overexpressed in 60~80% of all cases. Cyclin D1 overexpression was strongly correlated to estrogen receptor positivity and better histologic grade in breast cancer. The aim of this study was to correlate cyclin D1 overexpression using a well characterized antibody with estrogen receptor status and other clincopathologic parameters. METHODS: From March 1989 to December 1994, 85 patients with primary breast carcinoma were the subject in this study. We analyzed cyclin D1 expression by immnohistochemical staining using cyclin D1 antibody, cells were considered positive according to distinct nuclear staining. The correlation between cyclin D1 expression was compared with important clinicopathologic parameters (tumor size, axillary lymph node status, p53 expression, c-erbB2 expression, histologic grade, estrogen receptor status). RESULTS: Cyclin D1 expression was detected in 37 cases (43.5%). Cyclin D1 expression was high in patients with tumors that expressed estrogen receptor (58.5% vs 26.5%, P=0.019). Cyclin D1 was mainly overexpressed in the histologic grade I and II (75.0%), as compared with 65.2% in cyclin D1 negative tumor, however there was no statistical significance (P=0.067). There were no significant correlation with tumor size, axillary lymph node status, p53 expression, or c-erbB2 expression (P>0.05). CONCLUSION: Cyclin D1 expression in estrogen receptor (ER) positive patients was significantly higher than that seen in ER negative patients. There was a negative correlation between cyclin D1 and tumor histologic grade, however it was not statistically significant. Tumor size, axillary lymph node status, p53 expression, and c-erbB2 expression were not correlated with cyclin D1.