This article focuses on pharmacogenetic associations between genetic polymorphism of uridine diphosphate glucuronosyltransferase (UGT) 1A1 gene and irinotecan toxicity. Accumulating evidence provides support to the idea that determination of UGT1A1 polymorphisms before irinotecan treatment is clinically useful and important for predicting and avoiding related toxicities. On the basis of these backgrounds, the irinotecan label was updated in 2005 in the United States to provide pharmacogenetic information, and a dose reduction of irinotecan should be considered for patients known to be homozygous for the UGT1A1*28 allele when administered in combination with other agents or a single agent. The irinotecan/UGT1A1 issue and the development of molecular diagnostic testing are now to be translated into clinical practice.