This 2 arm study will assess the efficacy and safety of Avastin in combination with irinotecan + 5-fluorouracil/folinic acid, versus irinotecan + fluorouracil/folinic acid alone, as first line treatment in Chinese patients with metastatic colorectal cancer. Patients will be randomized 2:1 to receive 6-weekly cycles of Avastin (5mg/kg iv every 2 weeks) + irinotecan 125mg/m2 iv / leucovorin 20mg/m2 iv / fluorouracil 500mg/m2 iv weekly for 4 weeks, or 6-weekly cycles of irinotecan 125mg/m2 iv / leucovorin 20mg/m2 iv / fluorouracil 500mg/m2 iv weekly for 4 weeks. The anticipated time on study treatment is until disease progression, and the sample size is 100-500 individuals.

Inclusion Criteria:

• adult patients, >=18 years of age;
• histologically confirmed adenocarcinoma of the colon or rectum, with metastatic disease;
• >=1 measurable lesion;
• ECOG performance status of <=1.

Exclusion Criteria:

• prior systemic therapy for advanced or metastatic disease;
• adjuvant or neo-adjuvant treatment for non-metastatic disease in past 6 months;
• other malignancy within past 5 years, except cured basal cell cancer of skin or cured cancer in situ of cervix;
• clinically significant cardiovascular disease in past 6 months.