OBJECTIVES: To establish the current level of resource utilisation required to monitor International Normalised Ratio (INR) levels in patients treated with warfarin in the UK National Health Service. METHODS: Monitoring services in areas associated with six secondary care Trusts were studied. Accurate descriptions of services were obtained through one-to-one semi-structured interviews with NHS staff involved in the delivery of anticoagulant monitoring. Due to the complexity of the different systems, where possible, resource use was divided into the 3 main steps: taking blood (including transport of patients and/or bloods), INR analysis, and the communication of results and warfarin dose changes. Costs associated with running the service were identified and sourced from local settings or NHS reference costs as appropriate (2004 prices). RESULTS: The six monitoring services were responsible for an average of 3,459 (Range 1,422-4,500) warfarin patients, with a mean frequency of INR monitoring of 14.2 times per year (SD 3.4). The average cost of a monitoring visit was £14.58 (SD 4.25), of which £6.88 as associated with taking bloods, £4.08 with analysis and £3.62 with communication of results and dose changes. The mean annual cost per patient of INR monitoring was £206.41 (SD 63.51). Three of the services had separate hospital and primary/shared care models of INR monitoring. In these services the cost per INR test tended to be lower in the hospital based model (Mean £13.39) than in models that included primary care (Mean £23.06). CONCLUSIONS: The cost of INR monitoring varies according to setting and system. Where monitoring involves primary care, it tends to be more costly than systems in which hospital anticoagulation clinics control monitoring and warfarin treatment. This may be due to economies of scale and the ability of specialist clinics to deal with the monitoring process more efficiently.