Since important candidate UGT1A polymorphisms, including UGT1A1*6, UGT1A1*27, UGT1A1*28, UGT1A1*60, UGT1A7, and UGT1A9*22, may play key roles in irinotecan metabolism, we evaluated the frequencies of UGT1A polymorphisms and their association with toxicity and other parameters in Korean gastric cancer patients. The gene discussed is UGT1A9; the disease is gastric cancer.