The significant association between severe diarrhoea and the UGT1A1 genotype in our study is attributed to focusing on 1312 patients with advanced colorectal cancer who received three irinotecan-based regimens, supporting that the UGT1A1 genotype may serve as a predictive marker for irinotecan-induced severe diarrhoea. Here, UGT1A1 is linked to colorectal cancer.