OBJECTIVES: To identify patient characteristics associated with filling a thienopyridine prescription (clopidogrel or prasugrel) following acute coronary syndrome (ACS) and percutaneous coronary intervention (PCI) in US health plans. METHODS: We identified patients hospitalized for ACS-PCI with continuous enrollment in MarketScan databases for ≥ 6 months before admission and 60 days after discharge, without history of stroke or transient ischemic attack. Pharmacy dispensing records were searched to determine the first, if any, thienopyridine dispensed within 60 days after discharge. Demographic characteristics, medical and treatment history during the 6 months prior to the hospitalization were compared for patients filling prescriptions of clopidogrel, prasugrel, or neither. RESULTS: A total of 39,953 patients were eligible for the analysis; 29,900 (74.8%) filled clopidogrel prescriptions, 5,243 (13.1%) filled prasugrel prescriptions, and 4,810 (12.0%) filled no thienopyridines within 60 days after ACS-PCI. Patients filling prasugrel were younger (mean age 55.9 vs. 60.9, P<0.0001), more likely to be male (78.9% vs. 72.2%, P<0.0001), and from the Midwest (38.3% vs. 33.9%, P<0.0001) compared with clopidogrel users. Patients filling prasugrel had higher percentage of ST elevation myocardial infarction (STEMI) during the hospitalization than clopidogrel (40.7% vs. 33.3%, P<0.0001). Charlson Comorbidity Index (CCI) was low (0-1) for more prasugrel than clopidogrel users (85.9% vs. 80.2%, P<0.0001); prevalence of many comorbidities was also lower among prasugrel users. In contrast, patients with no thienopyridine after ACS-PCI were older (mean age 62.2), more likely to have high (≥2) CCI (25.5%, P<0.0001), less likely to have STEMI (25.6%, P<0.0001) and a higher prevalence of many comorbidities than patients taking clopidogrel. CONCLUSIONS: Patients filling no thienopyridine prescription following ACS-PCI were older and had more comorbidities than patients filling clopidogrel prescriptions. Patients filling prasugrel prescriptions tended to be younger and have fewer comorbidities than those with clopidogrel, which may reflect lower bleeding risk of this ACS-PCI population.