As the gene polymorphism of uridine diphosphate glucuronosyl transferase 1A1(UGT1A1)is related to the side effect of diarrhea induced by irinotecan. UGT1A1 gene *28 (6/6 and 6/7) and *6 (G/G and G/A) is related to low probability of diarrhea and UGT1A1 gene *28 (7/7) and *6 (A/A)is related to high probability of diarrhea. The purpose of this study is to find out the efficacy and side effect between two different dosages of irinotecan combined with cisplatin scheme in extensive disease-small cell lung cancer with UGT1A1 gene *28 (6/6 and 6/7)and *6 (G/G and G/A), based on the hypothesis that the UGT1A1 gene *28 (7/7) and *6 (A/A)is few in the Chinese population and increasing the dose of irinotecan can improve the efficacy without increasing the side effect in the patients with UGT1A1 gene *28 (6/6 and 6/7)*6 (G/G and G/A).