Background: The uridine diphosphate glucuronosyltransferase (UGT) 1A1, which transforms SN-38 into SN-38 glucuronide, is a key enzyme involved in the metabolism of irinotecan. Previous studies showed that UGT1A1 genotype is related to the toxicity of irinotecan-based chemotherapy in metastatic colorectal cancer. The purpose of this study was to investigate the relationship between UGT1A1genotype and safety and efficacy of irinotecan monotherapy in patients with advanced gastric cancer. Methods: We reviewed the data of 208 patients who were tested for UGT1A1 genotype and treated with irinotecan-based chemotherapy for advanced gastric cancer from 2009 to 2014. We evaluated the efficacy and safety of irinotecan monotherapy in the three groups with wild-type (WT), single heterozygosity (SH), and homozygosity/double heterozygosity (Homo-DH) classified by the genotypes for UGT1A1*28 or UGT1A1*6. Results: A total of 117 patients received irinotecan monotherapy: 40 patients in second-line, 74 in third-line, and 3 in forth-line therapy. The UGT1A1genotype was WT in 62 patients (53.0%), SH in 41 (35.0%), and Homo-DH in 14 (12.0%). Patients’ characteristics were similar among the three groups. The initial dose of irinotecan was reduced in 10 patients (16%) with the WT genotype, in 11 (27%) with SH, and in 10 (71%) with Homo-DH. Grade 3-4 neutropenia, diarrhea, and febrile neutropenia occurred in 13/22/64%, 6/5/21%, and 2/7/50% of WT/SH/Homo-DH patients. Median time to treatment failure of second-line and third-line therapies were 2.4/2.8/3.3 months and 2.4/2.3/1.3 months in WT/SH/Homo-DH patients. Median overall survival of second-line and third-line therapy were 7.9/9.9/4.6 months and 6.9/6.3/2.8 months in WT/SH/Homo-DH patients. Conclusions: Patients with UGT1A1 Homo-DH displayed high frequency of grade 3-4 toxicities, although the initial dose of irinotecan was reduced in some patients. UGT1A1 polymorphism may be related to the efficacy of irinotecan monotherapy in second- and third-line treatments for advanced gastric cancer.