Uridine 5′-diphospho-glucuronosyltransferase (UGT), a metabolic enzyme of irinotecan active metabolite, has two genetic polymorphisms (UGT1A1*6 and UGT1A1*28). In UGT1A1 homozygous or heterozygous patients, metabolism is delayed and the risk of developing adverse effects is increased, and therefore, dose reduction of irinotecan is considered. However, the specific dose reduction rate of irinotecan for heterozygous patients is uncertain. We studied the necessity of irinotecan dose reduction and its optimal dose in UGT1A1 heterozygous patients with lung cancer. Patients with lung cancer treated with irinotecan in the Tokushima University Hospital or Tokushima Municipal Hospital were included in this study. The dose of irinotecan was evaluated based on the relative dose intensity (RDI). The time to treatment failure (TTF) was defined as the period until treatment change, death, or progressive disease based on response evaluation criteria of solid tumors. We targeted 31 patients treated with irinotecan: 12 wild types (WT), 14 heterozygotes, and 1 complex heterozygote and 4 homozygotes. There was no significant difference in the TTF, but the mean RDI during the entire treatment period was significantly different in the wild type (79%), heterozygous (62%), and complex heterozygous and homozygous groups (46%). In addition, the proportion of patients who completed treatment without dose reduction in the WT group tended to be higher than that in the other groups. For lung cancer patients with UGT1A1 heterozygote types who start irinotecan therapy, reducing the initial dose by approximately 20% might be a safer chemotherapy without decreasing the therapeutic effect.