Background: The FOLFOXIRI regimen (irinotecan, oxaliplatin, fluorouracil [5-FU] and folinic acid [FA]) increased the response rate and overall survival compared to FOLFIRI in patients with metastatic colorectal cancer (mCRC). Adding cetuximab to FOLFOX or FOLFIRI increased efficacy in patients with k-ras wild type mCRC. We explored the dose limiting toxicity and feasibility of the combination cetuximab, irinotecan, oxaliplatin, 5-FU and FA in mCRC patients.Methods: In a dose-escalation study patients with previously untreated mCRC and a WHO performance status 0-1 received cetuximab (500 mg/m², 2 h), followed by irinotecan (95, 125, and 165 mg/m² in the dose levels [DL] 1, 2, and 3 respectively), followed by oxaliplatin (85 mg/m², 2 h) which was given parallel to FA (400 mg/m², 2 h) and followed by 5-FU (3200 mg/m², 46 h) in an outpatient setting every two weeks. The primary endpoints were the maximum tolerable dose and the safety. The trial was approved by the local ethics committee.Results: From 2007 to 2008, twenty patients were treated in this trial. In the first dose level (irinotecan 95 mg/m²) one patient developed neutropenia grade 4. One patient experienced diarrhoea grade 3 as DLT in dose level 2 (irinotecan 125 mg/m²). In dose level 3 (irinotecan 165 mg/m²), three patients experienced a DLT (diarrhoea grade 3 and two patients with neutropenia grade 4). Thus, the recommended dose for a phase II trial is 125 mg/m² irinotecan in combination with oxaliplatin, 5-FU/FA and cetuximab. Most common grade ≥3 toxicities were neutropenia (40%), diarrhoea (25%) and acne-like rash (15%). No therapy associated death occurred.The confirmed overall response rate in all cohorts was 75% (95%-CI 51-91%). The best response was reached after a median of 3.0 (95%-CI 2.2 to 3.7) months. Median progression free survival (PFS) is 16 (95%-CI 12.6-19.4) months, overall survival (OS) 33 (95%-CI 26.2-39.8) months.Conclusions: The combination of cetuximab and FOLFOXIRIis feasible and has an acceptable toxicity profile in patients with a good performance status. The observed clinical activity with a confirmed response rate of 75% is promising and further evaluated in the ongoing CELIM2.Trial registration: http://www.clinicaltrials.gov: NCT00422773.