Background: Superiority of irinotecan (CPT-11) plus cisplatin over etoposide plus cisplatin in small cell lung cancer (SCLC) has recently been demonstrated. This study determines dose-limiting toxicity (DLT) and maximum tolerated dose (MTD) of escalating doses of carboplatin to a fixed dose of irinotecan in Caucasians with small cell lung cancer. Patients and Methods: Patients with extensive small cell lung cancer received 50 mg/m² irinotecan on day 1, 8, and 15. Dose escalation of carboplatin on day 1 started in dose level 1 at an AUC of 5 mg × min/ml and was escalated to AUC 6 in level 2. Cycles were repeated on day 29. Dose escalation was evaluated after 3 consecutive patients. If no grade IV neutropenia lasting for ≧7 days or thrombopenia or non-hematologic toxicity ≧ grade III occurred, treatment was continued in the next dose level. Results: 16 patients were treated. DLT was reached in dose level 2 with 2 grade IV neutropenias and 1 grade IV thrombopenia and diarrhea. Toxicity was further investigated at dose level 1 in a total of 10 patients, which revealed 2 grade III neutropenias and 1 grade III diarrhea. Of 15 evaluable patients, 10 had a partial response, 3 had disease stabilization and 2 progressed. Conclusion: Dose level 1 was found to be MTD, this dose is currently compared to the combination of etoposide plus carboplatin within a randomised phase II/III trial.