Target molecules for a highly sensitive and specific diagnosis of breast cancer in its early clinical stages have not yet been identified. Here, we show the first evidence for diagnostic performance of the molecule B cell-specific Moloney murine leukemia virus integration site 1 (Bmi-1) in breast cancer patients. Only 5 out of 46 non-cancerous samples were positive for Bmi-1 mRNA expression resulting in a sensitivity and specificity of 72.0 and 91.3%, respectively. The mRNA expression was estimated using the cut-off value obtained from the receiver operating characteristic curve analysis. Further, Bmi-1 mRNA expression was found to be elevated in 97.8% (45/46) of cancerous tissues in comparison to the expression in paired cancerous tissues and non-cancerous tissues obtained from identical patients. Bmi-1 mRNA was found to be highly expressed even in the early clinical stages of breast cancer. Our results suggest that Bmi-1 mRNA might be a new tool to support the diagnosis of breast cancers, irrespective of the clinical stage.