Purpose: The purpose of this study is to determine the association between the BIK/NBK gene expression and estrogen receptor alpha expression. Materials and methods: We determined the association of BIK/NBK gene expression by real time quantitative reverse transcription polymerase chain reaction and estrogen receptor alpha expression by immunohistochemistry in samples of breast cancer tissue. Results: We found a statistically significant correlation of BIK/NBK gene expression with the estrogen receptor alpha expression (ρ = 0.751, p = 0.004). For verify differences of BIK/NBK gene expression among ERα+ and ERα- breast cancer tissues, Mann-Whitney U test was performed, obtaining significant differences. Conclusions: BIK/NBK gene expression may have important clinical implications and provide predictive, prognostic or therapeutic marker in breast cancer patients according to the estrogen receptor alpha expression.