We encountered a 64-year-old UGT1A1*28 heterotype patient who received CPT-11 and CDDP chemotherapy for Stage III A small-cell lung-cancer and developed grade 4 neutropenia as assessed by CTCAE v3.0 in Pharmaceutical Management. The therapeutic effect was good; 4 courses of chemotherapy could be safely administered at 1/3 of the applied dose of CPT-11, and CR was obtained. In this period, we participated in selecting antibiotics to neutropenia and CPT-11 closing regimen as pharmacists. Additionally, we carefully monitored patients about their bowel pattern and hematotoxicity, and collected information about gene searching for the patient and the Hospital Ethics Committee. The analysis of the gene polymorphism of CPT-11 is useful to predict the occurrence of serious side effects, but this judgment cannot easily be made clinically. In the event of a marked decrease in the neutrophil count of a patient during the first courses of treatment with CPT-11, we believe that it is important to consider gene polymorphism, to closely monitor the symptoms and laboratory parameters, and to give a smaller dose of the drug.