A phase I study was conducted to determine dose-limiting toxicity (DLT) and maximum tolerated dose (MTD) of carboplatin combined with irinotecan and docetaxel on a divided schedule with recombinant human granulocyte colony stimulating factor (rhG-CSF) support in patients with stage IIIB or IV non-small cell lung cancer. Carboplatin was given at the dose of AUC5 on day 1. Irinotecan and docetaxel on days 1 and 8 were administered at a starting dose of 40 and 30 mg/m2as level 1. Subsequent levels were: irinotecan/docetaxel (in mg/m2), 50/30 (level 2), 60/30 (level 3) and 60/35 (level 4). rhG-CSF was given at 50 mg/m2on days 5–15. Cycles were repeated every 3 weeks. Between May 1999 and April 2001, 31 patients were registered in this phase I study. Level 4 was judged as the MTD. The DLTs were considered diarrhea and febrile neutropenia. The overall response rate was 32.3% and median survival was 490 days with 1-year survival of 65.1%. We conclude that both irinotecan 60 mg/m2and docetaxel 30 mg/m2on days 1 and 8 in combination with an AUC5 of carboplatin on day 1 with rhG-CSF support is recommended for phase II study. The response rate and survival data in this phase I study are encouraging. We considered that the pathogenesis of diarrhea involved not only direct cytotoxic damage to the mucosa, but also bacterial overgrowth.