OBJECTIVES: Anticoagulation with warfarin for stroke prevention requires careful management to avoid hemorrhage or thrombosis. We evaluated the frequency of international normalized ratio (INR) testing in two high-risk Medicare populations who are likely to receive adjusted-dose warfarin: 1) patients diagnosed with atrial fibrillation (AF); and 2) patients with a mechanical heart valve (MHV). METHODS: We analyzed the 2001 Physician Supplier Procedure Summary Master Files (PSPSMF) database, a 5% sample of procedure-specific claims for all physician/supplier services rendered to Medicare beneficiaries. We identified patients with AF and MHV based on any-listed ICD-9-CM diagnosis code of 427.31 and V43.3, respectively. For each cohort, we searched for claims billed under Current Procedural Terminology (CPT) code 85610 – prothrombin testing. We compared prevalence and frequency of INR testing for each cohort to expected standards of care. RESULTS: We identified 141,757 patients with AF and 10,055 patients with MHV, which would yield projected national estimates of 2.8 million and 200,000 patients, respectively. Sixty percent of AF patients and 43% of MHV patients did not have a single INR claim. Of AF patients who had at least one INR test claim, 41% were tested less than 6 times per year, and 59% were tested 10 or fewer times per year. For MHV patients, 31% were tested less than 6 times per year, and 47% were tested 10 or fewer times per year. CONCLUSIONS: Medicare claims histories reveal that clinical practice patterns may not adhere to accepted standards of care for the prevention of stroke in AF and MHV. Third-party payment policies, provider behavior, lack of patient awareness, and other factors may contribute to poor compliance and possible adverse events. Additional studies are needed to determine the cause of under-compliance in managing patients at risk for stroke and how Medicare policies may affect prescribing decisions and patient outcomes.