We included studies that met the following criteria: (a) participants were receiving aspirin therapy as an antithrombotic; (b) participants were classified prospectively as aspirin sensitive or aspirin resistant before the ascertainment of any clinical outcome, or were grouped on the basis of clinical outcome and then classified for aspirin status (we considered patients to be aspirin sensitive if their platelets responded as expected to aspirin treatment, and platelet function, however measured, was inhibited, and we considered patients to be aspirin resistant if their agonist induced platelet response was not inhibited by aspirin as expected); (c) there was adequate allocation concealment such that investigators were blinded to the patients’ aspirin sensitive and aspirin resistant status; (d) a measure of prospective clinical outcome was used in both groups of patients; and (e) patients receiving other antiplatelet treatment were also included. (The expectation was that there would be an attenuation of the rate of any adverse effects associated with aspirin resistance in the presence of a second platelet inhibitor, given the perceived rationale for prescribing the second antiplatelet agent. Alternatively, if no attenuation of any adverse events occurred in the presence of a second platelet inhibitor, this could possibly reflect a lack of any benefit of the second platelet inhibitor.)