This randomized, open-label study will evaluate the safety and efficacy of Avastin (Bevacizumab) added to standard mFOLFOX6 chemotherapy in treatment-naïve patients with Stage IV metastatic colorectal cancer. According to K-RAS gene mutation status, patients will be assigned or randomized to receive either Avastin 5 mg/kg intravenously (iv) on Day 1 of each 2-week cycle or cetuximab 400 mg/m2 iv on Day 1 followed by 250 mg/m2 iv every week, in addition to mFOLFOX6 every 2 weeks. Anticipated time on study treatment is until disease progression or unacceptable toxicity occurs.

Inclusion Criteria:

• Adult patients >/= 18 years of age
• Histologically confirmed adenocarcinoma of the colon or rectum
• Stage IV metastatic disease with at least one measurable metastatic lesion according to RECIST criteria
• Tumour tissue sample available for assessment of K-RAS and BRAF genes
• Prior radiotherapy must have been completed 4 weeks before randomization
• Adequate bone marrow, kidney and liver function
• Eastern Cooperative Oncology Group (ECOG) performance status 0-1

Exclusion Criteria:

• Previous chemotherapy for metastatic disease
• Completion of adjuvant treatment for colorectal cancer (Stage I, II and III) in the 12 months preceding randomization
• Prior treatment with bevacizumab, cetuximab or other EGFR inhibitors
• Clinical or radiographic evidence of brain metastases
• Clinically significant cardiovascular disease or disorder
• History of neoplastic disease other than colorectal cancer in the 3 years prior to start of study treatment, except for successfully treated non-invasive carcinomas such as cervical cancer in situ, basal cell carcinoma of the skin or superficial bladder tumours
• HIV, hepatitis B or C infection