The association between the UGT1A1*28 genotype and the severe toxicity of low-dose irinotecan has been controversial, and few studies have examined this association in patients with lung cancer. The aim of this study was to assess the association between the UGT1A1*28 genotype and the severe toxicity of low-dose irinotecan in Japanese patients with lung cancer. From December 2005 through July 2008, 53 Japanese patients with advanced lung cancer who underwent chemotherapy that included low-dose irinotecan (50 or 60 mg/m2) as a single agent or in combination chemotherapy were retrospectively analyzed. Genomic DNA was extracted from peripheral blood. Genotypes for the UGT1A1*28 were denoted as wild-type for 6/6, heterozygous for 6/7, or homozygous for 7/7 depending on the number of TA repeats found in each allele. Of the 53 patients, 42 (79.2%) were wild-type, 9 (17.0%) were heterozygous, and 2 (3.7%) were homozygous for the UGT1A1*28 genotype. The UGT1A1*28 genotype was not associated with grade 3 or 4 neutropenia, thrombocytopia, diarrhea, or febrile neutropenia. The frequency of dose reduction of irinotecan did not differ between wild-type and heterozygous or homozygous for the UGT1A1*28 genotype. In addition, there were no significant differences in response rates and survival between wild-type and heterozygous or homozygous for the UGT1A1*28 genotype. In conclusion, the UGT1A1*28 genotype did not predict the severe toxicity of low-dose irinotecan in patients with lung cancer. Therefore, low-dose irinotecan could be administered without reducing starting dose in patients with UGT1A1*28 genotype.