OBJECTIVES: : An economic evaluation was conducted to investigate the Incremental Cost-Effectiveness Ratio (ICER) of cetuximab in combination with FOLFIRI versus FOLFIRI, between three cohorts of the CRYSTAL study, and determine if the cost- effectiveness improves when treatment is stratified to patients with the genetic biomarkers, KRAS wild-type and RAS wild-type (wt)  METHODS: From the CRYSTAL study, Individual Patient Data (IPD) was obtained from Merck Serono Biostatistics department. It was categorised into the three cohorts: the Intention To Treat (ITT) population and the two subgroups KRAS wild-type and RAS wild-type. Survival analysis was conducted on this data using R studio. Adverse events and resection rates were also obtained for the cohorts. NHS acquisition costs for cetuximab were used. A Merck Serono Cost Utility Model was then re-engineered to economically evaluate the three cohorts for comparison.   RESULTS: From this analysis, the deterministic base case ICER, cost per Quality Adjusted Life Year (QALY) gained, results are £130,929 in the ITT, £72,053 in the KRAS wt and £44,184 in the RAS wt cohorts. CONCLUSIONS: From these results, it can be concluded that based on the data from the CRYSTAL study, stratification of patients by genetic biomarker KRAS wt and RAS wt does improve the cost effectiveness of cetuximab plus FOLFIRI versus FOLFIRI alone. The RAS wt cohort had the lowest ICER and is therefore the most cost effective of the three groups.