Several studies have demonstrated with the use platelet function tests (PFT) that subgroups of patients under acetylsalicylic acid (ASA) fail to produce the anticipated antiplatelet effect. This phenomenon as well as the clinical failure of ASA to protect patients from thromboembolic complications has been termed ASA resistance (AR) or ASA nonresponsiveness. Several subtypes of AR can be distinguished by PFT. The following PFT were used to characterize AR: optical aggregometry, platelet aggregation in whole blood, platelet function analyzer (PFA-100), platelet reactivity test or platelet aggregate ratio, flow cytometry and thromboxane B(2) generation. All PFT have in common that their widespread clinical use is substantially limited due to complex preanalytic factors, reduced specificity and poor reproducibility. PFT are not interchangeable for monitoring antiplatelet treatment. Three prospective clinical trials revealed a possible relationship between AR and subsequent cardiovascular events. There is a need for a simple and reliable assay for predicting the clinical efficacy of antiplatelet therapy. Recent data demonstrate that none of the currently available assays including the PFA-100 system are capable to accomplish these objectives.