BACKGROUND Cyclin D1 protein plays an important part in regulating the progress of the cell during the G(1) phase of the cell cycle. It has been suggested that G870A polymorphism at the exon4/intron4 splicing region of the CCND1 gene may play a role in tumorigenesis and invasiveness. METHODS A case-control study was performed to test the association between G870A polymorphisms in the CCND1 gene and breast cancer risk and cancer progression. For this purpose, 38 patients with breast cancer and 64 healthy women controls were included in the study. The CCND1 G870A polymorphisms in our study groups were genotyped by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) using peripheral blood samples. RESULTS A significant difference was found in the distribution of the GG, AG and AA genotypes between the patient group and the control group (p=0.021). A lower risk (odds ratio 0.435, 95% confidence interval 0.223-0.846) was found to be associated with heterozygote AG individuals when compared with homozygote allele carriers in breast cancer. The cyclin D1 A870G genotype was associated with capsular invasion (p=0.02). CONCLUSIONS The risk of breast cancer development and prognosis may be associated with genetic variation in the CCND1 genotype, which may be used as a biomarker for further studies.