Introduction: The risk of subacute thrombosis (SAT) after percutaneous coronary intervention (PCI) is high if patients show high whole blood platelet aggregation. Hypothesis: For such high-risk patients, even though dual antiplatelet therapy is performed, whole platelet aggregation should be quantitated and if high, the indication for additional antiplatelet therapy determined. Methods: This is a retrospective analysis of 26,314 stents from 26,314 patients (20,028 males; mean age 70±11 years). The whole blood platelet aggregatory threshold index (PATI; WBA-Neo; ISK, Osaka, Japan) was calculated. PATI was used to determine the minimum concentration of adenosine 5'-diphosphate causing a non-reversible aggregation of platelets and indicated platelet aggregation (range 0-8 μM). The higher the PATI, the lower the platelet aggregation. All patients received single or double antiplatelet drug therapy according to physician choice. Results: SAT occurred in 35 (0.13%) of 26,314 patients after PCI. Total blood PATI was 5.25±2.46 in 35 patients with SAT and 7.14±1.75 in 26,279 patients without SAT (P<0.001). Receiver operating characteristic (ROC) curves of the PATI for detection of occurrence of SAT showed an area under the curve (AUC) of 0.721. Best cutoff point of PATI was 6.435μM (sensitivity of 81% and specificity of 66%) for detection of occurrence of SAT (Youden index=0.464, P<0.001). No significant differences existed in the percentage of each coronary risk factor between patients with and without SAT after PCI (63% vs 47% for diabetes mellitus, P=0.054, 66% vs 75% for hypertension, P=0.209, 46% vs 57% for hyperlipidemia, P=0.190, and 34% vs 28% for smoking habits, P=0.381). Conclusions: In patients after PCI, despite single or double antiplatelet drug therapy, the whole blood platelet aggregation should be checked, and if PATI is <6.435 the indication for additional antiplatelet therapy should be considered.