Estrogen-related receptors (ERRs) alpha, beta, and gamma are orphan nuclear receptors that modulate the estrogen signaling pathway and play roles in the regulation of breast cancer cell growth. To determine the association between breast cancer risk and alleles of the tetranucleotide repeat (AAAG)n in the intron of ERRγ gene, a case-control study of 200 breast cancer patients and 200 controls was performed in Iranian women. Our results demonstrate that women with short AAAG repeat are at higher risk of breast cancer (OR 7). This result suggests a possible involvement of polymorphic AAAG repeat of ERRγ gene in regulating its expression.