Patients with the UGT1A1*6 polymorphism showed a significantly higher incidence of grade III–IV diarrhea (9/34 patients [26.47%], p = 0.012) and grade III–IV neutropenia (14/34 patients [41.18%], p = 0.044) after the irinotecan-containing regimen. Here, UGT1A1 is linked to neutropenia.