The phase III AXEPT study showed the noninferiority of modified capecitabine plus irinotecan (mXELIRI) with or without bevacizumab relative to fluorouracil, leucovorin, and irinotecan (FOLFIRI) with or without bevacizumab as a second-line treatment for metastatic colorectal cancer. We evaluated the associations between the UGT1A1 genotype linked to adverse events-caused by irinotecan-and the efficacy and safety of mXELIRI and FOLFIRI. The UGT1A1 genotype was prospectively determined and patients were categorized into three groups according to WT (*1/*1), single heterozygous (SH; *28/*1 or *6/*1), and double heterozygous or homozygous (DHH; *28/*28, *6/*6, or *28/*6). Overall survival (OS), progression-free survival, response rate, and safety were assessed. The UGT1A1 genotype was available in all 650 randomized patients (WT, 309 [47.5%]; SH, 291 [44.8%]; DHH, 50 [7.7%]). The median OS was 15.9, 17.7, and 10.6 months in the WT, SH, and DHH groups, respectively, with an adjusted hazard ratio (HR) of 1.53 (95% confidence interval [CI], 1.12-2.09; P = .008) for DHH vs WT or SH. The median OS in the mXELIRI and FOLFIRI arms was 18.1 vs 14.3 months (HR 0.80; 95% CI, 0.62-1.03) in the WT group, 16.3 vs 18.3 months (HR 1.04; 95% CI, 0.79-1.36) in the SH group, and 13.0 vs 9.1 months (HR 0.71; 95% CI, 0.39-1.31) in the DHH group, respectively. Modified capecitabine plus irinotecan with or without bevacizumab could be a standard second-line chemotherapy in terms of efficacy and safety regardless of the UGT1A1 genotype.