This study reports the frequency of aspirin resistance and its correlation with clinical and biochemical parameters among 280 healthy Turkish volunteers (179 men, 101 women) who were taking 100 mg of aspirin 7 days or more. Aspirin resistance was detected by optical platelet aggregometry, using adenosine diphosphate and arachidonic acid, and defined as a mean aggregation of 64% or more with 5 mu M adenosine diphosphate and a mean aggregation of 20% or more with 0.5-mg/mL arachidonic acid. Of the study population, 27.5% (26 women [25.5 %] and 51 men [28.5 %]) were aspirin resistant. The current findings indicate that aspirin resistance is an important and real laboratory diagnosis given its frequency of 27.5% in the study population. These results of this large trial evaluating the frequency of aspirin resistance in healthy subjects indicate that aspirin resistance should be diagnosed so that individuals with no response can receive alternative or additional antiplatelet therapy.