In this study, although there were no significant differences between grade 3–4 neutropenia and patients with UGT1A1*28, the incidence of severe neutropenia in patients with hetero- and homozygous *28 was higher than patients with homozygous wild type at the first cycle (OR 2.7, 95% CI 0.8–8.8; P = 0.087). Here, UGT1A1 is linked to Decreased total neutrophil count.