OBJECTIVE: To estimate the average cost of anticoagulation clinic services from a provider perspective. METHODS: Using a retrospective cohort approach, a random sample of 600 patients was selected from 3 geographically diverse managed care sites (N=200 per site) with anticoagulation clinics and followed for up to one year. Care was provided under a traditional nurse or pharmacist managed clinic model with most encounters involving telephone contacts. Study patients included adults aged 18+ years with a diagnosis of nonvalvular atrial fibrillation who received warfarin for at least 30 days. Patient contacts were evaluated based on complexity (education, warfarin dose adjusted, warfarin dose not adjusted) and the total cost of anticoagulation clinic care was calculated for staff time, laboratory tests, and overhead. Unit costs were estimated based on national data. Labor costs were measured by observing the actual time spent by clinic staff on individual patient encounters using a prospective time study ("activity-based" approach), and through a survey of budgeted labor hours ("budgeted-cost" approach). RESULTS: The average age of patients was 72 years and 56% were female. Approximately 85% were white. About 75% of patients completed one year of clinic follow-up (mean duration: 10.5 months). Patients averaged 18 clinic contacts. The cost of staff time for warfarin monitoring during follow-up was estimated to be $91 under the activity-based approach and $176 using the budgeted-cost approach. These costs increased to $244 and $330 for the activity-based and budgeted-cost approaches, respectively, once laboratory tests and overhead were included. Average costs for patients who were followed for a full year were $268 and $362, respectively. While anticoagulation control was relatively similar across sites (60% to 65% time in INR range), there was more marked variation in costs. CONCLUSIONS: This study provides a detailed, multi-site assessment of the costs of warfarin monitoring using multiple costing methodologies.