Douillard et al. (1999), have reported a multicenter, randomized, controlled open label study comparing irinotecan (Campto®) in combination with 5-fluorouracil (5-FU) and folinic acid (FA) therapy with fluorouracil/FA alone as first line treatment for metastatic colorectal cancer. They concluded that irinotecan in combination with 5-FU offered consistent significant advantages in terms of efficacy and clinical benefit over single agent 5-FU without any detrimental effects upon quality of life. We aim to relate these data to relevant costs within the UK and to evaluate the economic implications of the difference in survival between the two treatment arms from the viewpoint of a UK NHS commissioner. This work develops the second line assessment of irinotecan by Iveson et al. (1999) in the EJC, which concluded that irinotecan achieved results of £14,000 per life year saved in the second line setting. This cost-effectiveness analysis compares the economic implications of replacing 5-FU therapy as a single agent (de Gramont regimen or AIO regimen) plus folinic acid rescue with irinotecan in combination with 5-FU/FA, (de Gramont regimen, or AIO regimen). Drug acquisition costs are derived from the British National Formulary (March 1999); unit costs for clinical consultations and services are derived from relevant 1997/1998 cost databases. Costs associated with treatment delivery and disease complications are also considered. Indirect costs although important are not included, in line with the viewpoint of commissioners within the NHS. Although cumulative drug acquisition costs per patient are higher with irinotecan and 5-FU/FA than with infusional 5-FU/FA therapy alone, these costs are at least partially offset by lower cumulative costs per patient associated with treatment of complications in the irinotecan plus 5-FU/FA arm than in the 5-FU/FA alone arm.