Aldehyde dehydrogenase 1 (ALDH1), also known as aldehyde dehydrogenase 1 family, is composed of six enzymes that are expressed at high levels in stem cells and are involved in the regulation of stem cell function. Increased ALDH1 activity has been found in the stem cell populations of leukemia and some solid tumors including breast cancer (BC). However, which ALDH1's isoenzymes are contributing to ALDH1 activity has not been determined. In addition, the prognostic value of individual ALDH1 isoenzyme is not clear. In the current study, we investigated the prognostic value of ALDH1 isoenzymes in BC patients through "the Kaplan-Meier plotter" (KM plotter) database, in which updated gene expression data and survival information are from a total of 3455 BC patients. ALDH1A1 messenger RNA (mRNA) high expression was found to be correlated to worsen overall survival (OS) for all BC patients. ALDH1A2 and ALDH1L1 mRNA high expressions were found to be correlated to better OS for all BC patients. Both of ALDH1A3 and ALDH1B1 mRNA high expressions were not found to be correlated to OS for all BC patients. These results strongly support that ALDH1A1 was only a biomarker for predicting poor survival of BC patients among ALDH1 isoenzymes. ALDH1A1 might be a major contributor of ALDH1 activity in BC, since only ALDH1A1 mRNA high expression was found to be significantly correlated to worsen OS for all BC patients.