Objective To investigate the clinical significance of the amplification of CyclinD1 gene and overexpression of its protein product in breast cancer. Methods Semi-quantitative PCR and immunohistochemical techniques were used to examine the amplification of CyclinD1 gene and overexpression of its protein product in breast cancer, mammary tissues adjacent to the tumor, mammary benign disease and normal mammary tissues. Results The results showed that amplification of CyclinD1 gene was observed in 14 out of 62 (22.6%) breast cancer, the overexpression of its protein product was 48.4% (30/62). Compared with other mammary tissues, these findings were significant. Amplification of CyclinD1 gene and overexpression of its protein were positively associated with histological grade, and there was a b correlation between CyclinD1 protein overexpression and estrogen receptor (progestogen receptor). Conclusion The degree of CyclinD1 protein overexpression is not quite the same as the frequency of the amplification of CyclinD1 gene. It suggests that the overexpresstion of CyclinD1 protein is also attributed to some other mechanisms. Estrogen might stimulate the expression of CyclinD1. The detection of CyclinD1 gene and its protein in breast cancer has some clinical significances.