['Aldehyde dehydrogenase 1 (ALDH1) is an important marker of tumor-initiating cells. We aimed to investigate ALDH1 expression in benign breast disease and human breast cancer of different histologic stages.', 'Immunohistochemical staining of ALDH1 was applied to 21 cases with benign breast diseases, 47 ductal carcinoma in situ (DCIS) cases, 62 cases diagnosed with invasive cancer with extensive intraductal component (EIC), and 58 cases diagnosed with invasive cancer without EIC.', 'ALDH1 was expressed in tumor cells in 61.0% of 164 breast cancer cases, which was higher than that in benign breast disease (3/21) (P\u2009<\u20090.001). Of these 167 breast cancer cases, a significantly higher rate (54/58) of intratumoral ALDH1 expression was observed in invasive cancer without EIC cases than that in DCIS cases (19/46, one case not available) and invasive cancer with EIC cases (27/60, two cases not available) (P\u2009<\u20090.001). Interestingly, a significantly higher rate (22/48) of intratumoral ALDH1 expression in invasive component was observed than that in in situ component (7/48) in the same tumor (P\u2009=\u20090.001). In 47 DCIS cases, no significant association was observed between ALDH1 positivity and any clinicopathological parameter (all P\u2009>\u20090.05). However, ALDH1 positive invasive breast cancers were significantly more likely to be with large tumor size (P\u2009=\u20090.001), high grade (P\u2009<\u20090.001), and high Ki67 expression (P\u2009=\u20090.009).'] Does aldehyde dehydrogenase 1 expression correlate with the invasion of breast cancer?