Background: Colorectal cancer (CRC) is considered as one of the most common tumors that has a remarkable increase in frequency, and mortality, especially in developing countries. ALDH1 has been used as a stem cell marker and plays an important functional role in the initial differentiation and maturation of stem cells. The expression of ALDH1 by cancer stem cells (CSCs) has been demonstrated in various types of cancers.

Aim: This work aimed to evaluate ALDH1 expression in CRC and determine the correlation between ALDH1 expression and different clinicopathological features.

Material and method: This study included 60 formalin-fixed and paraffin-embedded different cases of CRC, in which ALDH1 immunostaining was performed for all cases.

Results: There was a highly significant positive statistical correlation between ALDH1 expression, and lymph node metastasis, tumor-infiltrating lymphocytes, modified dukes staging, and lymphovascular invasion (P-value ≤ 0.01). Also, there was a significant positive statistical correlation between ALDH1 expression, and both tumor site and TNM staging (P-value < 0.05). Other clinicopathological variables such as age, sex, tumor size, histopathological subtypes, and tumor grade showed no significant statistical correlation (P-value > 0.05).

Conclusion: High ALDH1 expression was associated with colonic tumors more than the rectal ones, lymphovascular invasion, advanced tumor staging, and lymph node metastasis. Our results suggest that ALDH1 seems to play an important role in the tumor invasion and lymph node metastasis and accordingly, it may work as a poor prognostic indicator in CRC. Moreover, it may provide a biomarker for the evaluation of the tumor's aggressiveness.