Background: Several randomized trials have indicated that combination chemotherapy applied in metastatic colorectal cancer (mCRC) does not significantly improve overall survival when compared to the sequential use of cytotoxic agents (CAIRO, MRC Focus, FFCD 2000-05). The present study investigates the question in bevacizumab-based first-line treatment including escalation- and de-escalation strategies. Methods: The AIO KRK 0110/ML22011 trial (ClinicalTrials.gov NCT01249638) is a two-arm, multicenter, open-label randomized phase III trial comparing the efficacy and safety of Cape-Bev versus CAPIRI-Bev in the first-line treatment of metastatic colorectal cancer. Patients with unresectable metastatic colorectal cancer, ECOG PS 0-1, will be assigned in a 1:1 ratio to receive either capecitabine 1250 mg/m² bid for 14d (d1-14) plus bevacizumab 7.5 mg/kg (d1) q3w (Arm A) or capecitabine 800 mg/m² BID for 14d (d1-14), irinotecan 200 mg/m² (d1) and bevacizumab 7.5 mg/kg (d1) q3w (Arm B). Patients included into this trial are required to consent to the analysis of tumor tissue and blood for translational investigations. In Arm A, treatment escalation from Cape-Bev to CAPIRI-Bev is recommended in case of progressive disease (PD). In Arm B, de-escalation from CAPIRI-Bev to Cape-Bev is possible after 6 months of treatment or in case of irinotecan-associated toxicity. Re-escalation to CAPIRI-Bev after PD is possible. The primary endpoint is time to failure of strategy (TFS). Secondary endpoints are ORR, OS, PFS, safety and quality of life. Conclusion: The AIO KRK 0110 trial is designed for patients with disseminated, but asymptomatic mCRC who are not potential candidates for surgical resection of metastasis. Two bevacizumab-based strategies are compared: one starting as single-agent chemotherapy (Cape-Bev) allowing escalation to CAPIRI-Bev and another starting with CAPIRI-Bev and allowing de-escalation to Cape-Bev and subsequent re-escalation if necessary. By January 2012, 79 of planned 516 patients have been enrolled.