Purpose: It is widely accepted that aldehyde dehydrogenase(ALDH) activity is a signature of breast cancer stem cells, andhigh activity has been reported to be associated with poor clinicaloutcome. The aim of this study was to assess the expressionof members of the ALDH family of isozymes in breast cancer tissuesand to evaluate the implications of the results. Methods: Weanalyzed paraffin-embedded tumor tissue from 160 patients withbreast cancer. Immunohistochemistry (IHC) staining was performedon the slides using antibodies against different ALDHfamily members. We collated the IHC results with patient clinicalcharacteristics and determined their prognostic value. In addition,we analyzed normal, hyperplastic, and carcinomatous tissues insitu to check their ALDH distributions. Results: All the testedALDH members were detected in the various tissue types, but atdifferent levels. Only ALDH 1A3 was found to be significantly associatedwith distant metastasis (p=0.001), disease-free survival(p<0.001), and overall survival (p<0.001). Conclusion: The levelof ALDH 1A3 in breast cancer tissue is a predictive marker of apoor clinical outcome.