The objective of this study was to identify possible risk factors associated with a lack of response to aspirin and clopidogrel treatments in patients with coronary or cerebral ischemic artery disease. A point-of-care analyzer, VerifyNow (Accumetrics, San Diego, CA), was used to measure adenosine-5-diphosphate and platelet P2YI2 receptor blockage to investigate the responses of a group of 197 patients to aspirin and/ or clopidogrel therapies (aspirin therapy, 178; clopidogrel therapy, 139; both drugs, 144). Of these 197 patients, 135 (68.5%) had coronary artery disease and 72 (31.5%) had ischemic cerebrovascular disease. Aspirin resistance was defined as an ARU (aspirin reaction units) > or =550, and clopidogrel resistance was defined as platelet inhibition <20%. Twenty-five of 178 aspirin users (14.0%) were resistant to aspirin, and 54 of 139 (38.8%) clopidogrel users were resistant to clopidogrel. The data indicate that low hemoglobin (Hb) level in aspirin users and high systolic and diastolic blood pressures in clopidogrel users are significantly related to treatment resistance (p < 0.05). The latter finding is possibly due to the greater adhesiveness and increased aggregability of platelets in hypertensive patients.