OBJECTIVES: To conduct a robust economic evaluation, it is necessary to describe current practice and associated costs. Available data on clinical pathways and cost of chemotherapy are clinical trial-based, which may not reflect UK National Health Service (NHS) practice. Practice-relevant costs of drug administration, patient monitoring and management of adverse events, required for a practice-relevant economic model, are not available. This study aimed to inform an economic evaluation by describing patient-based cost of NHS patients with advanced colorectal cancer (CRC) undergoing irinotecan-based chemotherapy. METHODS: Resource use data were collected from the medical records of 48 patients prescribed irinotecan-based (IrMdG) chemotherapy at a UK tertiary care centre. Using the hospital perspective, data were collected from starting chemotherapy until treatment ended. Unit costs were assigned, based primarily on NHS Reference Costs 2008/09. Data were analysed using descriptive statistics and variations around the costs were obtained. Predictors of cost were identified from a stepwise multiple regression analysis (ordinary least squares). RESULTS: Total cost for 48 patients was £598,765.54 (UK £ 2008/09). Mean cost per patient was £12,474.28 (95% CI: £11,233.24 – £13,715.32, median £13,307.82, range £3,024.48 – £21,276.18). Cost components comprised: chemotherapy drugs (36.9%), chemotherapy delivery (21.4%), pharmacy cost (15.0%), oncology appointments (9.5%), central line insertion (5.0%), management of complications and co-morbidities (5.1%), management of adverse events (4.9%), and imaging (2.2%). Significant predictors of increased cost (p<0.05) identified from the stepwise regression were: number of chemotherapy cycles received (adjusted R2 0.81), neutropaenia (adjusted R2 0.83), no prior chemotherapy (adjusted R2 0.85) and full dose chemotherapy (adjusted R2 0.86). CONCLUSIONS: This study provides the first data describing patient-based costs associated with current NHS practice in this patient group, derived from a pragmatic observational study with no trial protocol dictating practice. These data should be used in ensuing economic evaluations to ensure relevance to current clinical practice.