OBJECTIVES To determine incremental costs and life expectancy of adding bevacizumab (bev) to an irinotecan plus infusional 5-fluorouracil/leucovorin (5-FU/LV) regimen (FOLFIRI) for the first-line treatment of patients with metastatic colorectal cancer (MCRC) in the UK. METHODS We performed a decision analysis with a THREE-health-state area under the curve model to explore the effects of adding bev treatment to an existing FOLFIRI-based regimen. The model structure was based on a published model by National Institute of Technology Excellence (NICE), UK. Progression-free (PFS) and overall survival (OS) data were derived from clinical trials comparing bev+irinotecan and 5-FU/LV (IFL) with IFL alone. The impact on OS for bev+FOLFIRI was included through an indirect comparison of hazard ratios (HRs) between bev+mIFL versus bev+FOLFIRI (HR for death: 1.79; 95% CI: 1.12-2.88). Treatment effect for bev+FOLFIRI was maintained during the 40-month clinical follow-up. The FOLFIRI PFS was created by applying the HR from IFL versus FOLFIRI (HR for progression or death: 1.51, 95% CI: 1.16-1.97) to the IFL PFS. Outcomes included life years, QALYs, direct costs and incremental cost-effectiveness ratios (ICERs). A life-time horizon (8 years) was used. Cost and outcomes were discounted by 3.5% per annum. Both deterministic and probabilistic sensitivity analyses were performed. RESULTS The estimated discounted life expectancy for bev+FOLFIRI-treated patients was 2.62 years (undiscounted: 2.77 years) versus 1.58 years (undiscounted: 1.63 years) in the FOLFIRI arm. The discounted costs in the 2 arms were £49,798 and £23,698, respectively. The discounted ICER was £25,045 per life-year gained and £40,532 per QALY gained. Sensitivity analysis on key variables showed that assumptions of size (HR) and duration of treatment effect were the most influential factors for the ICER. CONCLUSIONS Bev+FOLFIRI versus current FOLFIRI-based treatment regimens for MCRC increases survival for these patients and is a cost effective treatment option.