Objective: To assess the expression of retinoic acid receptor beta 2 (RAR-β2) in breast tumor and to evaluate the relationship between RAR-β2 expression and tumorigenesis of breast cancer. Methods: Immu-nohistochemistry was used to detect the expression of RAR-β2 protein in specimens from 40 cases of breast cancer, 40 cases of atypical ductal hyperplasia, 40 cases of fibroadenoma and 20 cases of normal breast tissues. Reverse transcription polymerase chain reaction (RT-PCR) was used to detect the mRNA expression levels of RAR-β2 gene in 20 cases of breast cancer, 20 cases of atypical ductal hyperplasia, 20 cases of fibroadenoma, and 20 cases of normal breast tissues. Results: Immunohistochemical staining results revealed that the positive expression of RAR-β2 protein showed nuclear staining. The positive expression rate of RAR-β2 was 30% (12/40) in breast cancer, 17.5% (7/40) in atypical ductal hyperplasia, 87.5% (35/40)in fibroadenoma, and 95% (19/20) in normal breast tissues. The expression of RAR-β2 protein in breast cancer was significantly lower than that in normal breast tissues (X~2=26.30, P<0.001). The expression of RAR-β2 was not significantly different between atypical ductal hyperplasia and breast cancer (P>0.05). No correlation was found between the expression of RAR-β 2 protein and the tumor size, menopausal age, lymph node metastasis, clinical stage, histological grade, or protein expression of ER and PR in breast cancer tissues (P>0.05). Follow-up results showed that 3 out of 28 patients with negative RAR-β2 expression had visceral organ metastasis, but only one of the 12 RAR-β2 positive patients had osseous metastasis. RT-PCR analysis showed that the positive expression rate of RAR-β2 mRNA in breast cancer, atypical ductal hyperplasia, fibroadenoma and normal breast tissues was 25% (5/20), 35% (7/20), 85% (17/20) and 100% (20/20), respectively. The RAR-β2 mRNA expression rate in breast cancer was significantly lower than that in normal breast tissues (X~2=30.43, P<0.001). No significant difference in RAR-β2 mRNA expression was found between atypical ductal hyperplasia and breast cancer tissues (P>0.05). Conclusion: RAR-β2 gene may play a repressive role in the initiation of breast cancer, and the loss of the expression of RAR-β2 gene may be the initial step in breast carcinogenesis.