Background and Objectives. Previous studies have shown alterations in the cell cycle regulatory proteins in breast carcinomas. However, the results of these studies remain controversial. Cyclin D1 (CCND1) and p27(KIP1) (CDKN1B) are two essential regulators of cell cycle progression. This study aimed to investigate the associations of CCND1 A870G and CDKN1B C79T polymorphisms with breast cancer risk. Patients and Methods. Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) was used to determine the genotype and allelic frequencies of polymorphisms. Seventy-eight breast cancer patients and 84 age-matched healthy controls were included in the study. Results. Frequencies of CT genotype and T allele of CDKN1B were found to be higher in breast cancer patients than in controls (p=0.013, OR: 1.514 95% CI: 1.086-2.114.15; p=0.007, OR=1.496; 95% CI: 1.111-2.014, respectively). The frequency of AA genotype of CCND1 was decreased in hormone receptor- (estrogen and progesterone receptors) negative patients with breast cancer (p<0.049, OR=0.286; 95% CI: 0.071-1.142) Conclusions. Even though CDKN1B polymorphism appears to be an important predictive factor for breast cancer risk and CCND1 polymorphism may be a prognostic biomarker for breast cancer, further investigations with larger study groups are needed to fully elucidate the role of CCND1 and CDKN1B polymorphisms in the development and prognosis of breast cancer.