Aldehyde dehydrogenase 1 (ALDH1) has been identified as a marker of cancer stem cells in breast cancer (BC). Recent studies showed that ALDH1 expression is correlated with poor prognostic parameters and worse clinical outcome in BC. We evaluated ALDH1 expression by immunohistochemistry in a series of 217 invasive BCs and investigated the correlation between ALDH1 expression and clinicopathological parameters, molecular subtypes (luminal A, luminal B, human epidermal growth factor receptor 2 [HER2] type, and triple-negative BC [TNBC]), and patient survival. There was a significant association between ALDH1 expression and tumor grade (p < 0.001), i.e., the expression of ALDH1 was higher in high-grade tumors. ALDH1 expression was significantly associated with estrogen and progesterone receptor (ER and PR) negativity (p < 0.001) and HER2 positivity (p = 0.001). ALDH1 expression ratios were higher in HER2 type and TNBC. There was a statistically significant correlation between ALDH1 negativity and luminal A subtype (p < 0.001). The overall and disease free survival were shorter in ALDH1+ tumors, although without statistical significance. We confirm that ALDH1 is a potentially important, poor prognostic factor in BC, associated with high histological grade, ER/PR negativity and HER2 positivity. For more accurate results, ALDH1 expression should be evaluated in larger case series including various types/subtypes of BC.