Background: Prostatic carcinoma contains a small population of cancer stem cells (CSCs) that promote tumor growth, maintenance, and progression.  CSC markers could provide a prognostic tool for human malignancies. Therefore, CSCs may have a role in prostatic carcinoma. This study aimed to evaluate the expression patterns and significance of stem cell marker aldehyde dehydrogenase 1A1 (ALDH1A1) in prostatic adenocarcinoma. Methods: This is a retrospective study performed upon 60 cases grouped as; 10 cases (16.7%) of benign prostatic hyperplasia (BPH), 6 cases (10%) of high-grade prostatic intraepithelial neoplasia (HGPIN) and 44 cases (73.3%) of prostatic adenocarcinoma (PCa) of different Gleason scores. Immunohistochemistry was applied on formalin-fixed, paraffin-embedded blocks using ALDH1A1. The relation between ALDH1A1 expression and clinicopathologic parameters was assessed. Results: Low expression of ALDH1A1 was seen in all cases (100%) of BPH, 5/6 (83.3%) of HGPIN and 12/44 (27.3%) of PCa, while high expression was in 1/6 case (16.7%) of HGPIN and 32/44 (72.2%) of PCa cases. There were significant relations between the expression levels of ALDH1A1 in PCa compared with HGPIN and BPH cases (P<0.01). ALDH1A1 expression showed highly significant associations with pre-operative serum PSA level, Gleason score, Gleason Grade Group (P<0.01) and was significantly associated with perineural invasion and tumor stage (P<0.05). No significant relations were found between ALDH1A1 expression and patients` age, capsular invasion, lymphovascular invasion, depth of invasion (pT) or lymph node metastasis.  Conclusion: ALDH1A1 is a prostate CSC marker, could be involved in tumorigenesis and progression of prostatic adenocarcinoma