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).

Inclusion Criteria:

• Histologic or cytologic diagnosis of small-cell lung cancer
• Extensive-stage disease, defined as disease extending beyond one hemithorax involving contralateral mediastinal, hilar or supraclavicular lymph nodes, and/or pleural effusion.
• Males or females between 18 to 75 years
• No prior chemotherapy, if the surgery or radiotherapy has been administered, the interval is at least above four weeks.
• Performance status of 0-2 on the ECOG criteria. Expected survival is above three months.
• At least one unidimensional measurable lesion meeting Response Evaluation Criteria in Solid Tumors (RECIST. 2000).
• Patient compliance that allow adequate follow-up. Informed consent from patient or patient's relative.
• Adequate hematologic (neutrophil count >= 1,500/uL, platelets >= 100,000/uL), hepatic (transaminase =< upper normal limit(UNL)x2.5, bilirubin level =< UNL x 1.5), and renal (creatinine =< UNL) function
• The gene type of UGT1A1 *28 is 6/6 and 6/7.
• If female: childbearing potential either terminated by surgery, radiation, or menopause, or attenuated by use of an approved contraceptive method (intrauterine device [IUD], birth control pills, or barrier device) during and for 3 months after trial. If male, use of an approved contraceptive method during the study and 3 months afterwards. Females with childbearing potential must have a urine negative HCG test within 7 days prior to the study enrollment.
• No concomitant prescriptions including cyclosporin A, valproic acid, phenobarbital, phenytoin, ketoconazole.

Exclusion Criteria:

• Non small cell lung cancer and carcinoid
• Medically uncontrolled severe diarrhea in recent three weeks.
• Inability to comply with protocol or study procedures.
• Medically uncontrolled serious heart, lung, neurological, psychological, metabolic disease
• Second primary malignancy that is clinically detectable at the time of consideration for study enrollment.
• Pregnant or breast-feeding.
• Enrollment in other study within 30 days
• Brain metastasis with symptoms