This review focuses on a pharmacogenetic association between genetic polymorphism of UGT1A1 gene and severe adverse reactions to irinotecan. Although many studies used pharmacokinetic parameters as surrogate measures for predicting clinical outcomes of irinotecan chemotherapy, they have not produced consistent evidence. On the other hand, genotyping results of UGT1A1 gene appear to predict severe adverse reactions more straightforward than the pharmacokinetic parameters or the phenotypes of the enzymatic activity. A case-control study of Japanese cancer patients revealed that those with the variant UGT1A1 alleles were at significantly higher risk of severe adverse reactions to irinotecan, suggesting that the genotyping strategy would be clinically useful. Nevertheless, clinical importance of the pharmacogenetic testing should differ for different patient groups and for different clinical situations. We need to keep this issue in mind in applying the pharmacogenetic evidence in clinical practice.