Abstract: Objective To investigate the expression and clinicopathological significance of aldehyde dehydrogenase 1 （ALDH1） in breast cancer tissues and its adjacent tissues. Methods Breast cancer tissues of 71 cases and adjacent tissues of 29 cases were collected，and ALDH1 was detected by immunohistochemical Envision method and the clinicopathological features of the tumor were analyzed. Results The expression of adjacent tissues of breast cancer was localized in the nucleus and the expression of breast cancer tissues was localized in the cytoplasm. Some cancer tissues showed scattered expression，and others were diffusely expressed. The positive rate of ALDH1 in T1-T2 tumors was 51. 9%，which was significantly higher than that in T3 tumors（23. 5%）. The positive rate of ALDH1 in histological grade I tumors was 75. 0%，which was significantly higher than that in grade III tumors （55. 6%）. The positive rate of ALDH1 in patients with axillary lymph node metastasis was 42. 1%，which was significantly higher than that in patients without metastasis （30. 0%）. Conclusion The difference in the positivity of ALDH1 expression may be a marker of benign and malignant breast tumors. The expression of ALDH1 may be correlated with tumor growth，differentiation，and invasion.

Key words: breast cancer, ALDH1, tumor stem cells, clinical pathology