Background/Introduction: Cigarette smoking is a potent modifiable risk factor for coronary artery disease (CAD). However, little is known about alterations to prothrombotic state and platelet reactivity early after smoking cessation following percutaneous coronary interventions (PCI).Purpose: We investigated alterations to platelet reactivity, coagulation and markers of platelet, endothelial, inflammatory and coagulation activation in clopidogrel-treated patients with CAD after PCI before and after smoking cessation.Methods: Smoking patients aged 18 years or older at least 30 days after PCI were recruited and encouraged to quit the habit. At baseline and at 30 days, we measured platelet reactivity with VerifyNow system, thrombomodulin, P-selectin, platelet factor 4 (CXCL4/PF4), citrullinated histone H3 (H3cit) and cotinine level.Results: Among 117 patients, 84 patients (72%) at a median age of 60.5 years (40 [interquartile range 30–47] pack-years) completed a 30-day follow-up. At day 30, 30 (35.7%) patients stopped smoking with cotinine level < 50 ng/ml. Baseline characteristics were similar in both groups. In smoking quitters a change in platelet reactivity was larger (Δ platelet reactivity units (PRU) 19 [2, 43] vs. -6 [-32, 37], p = 0.018), along with a change in P-selectin concentration (-11.82 [-23.62, 1.34] vs. 7.19 [-14.24, 17.19] ng/ml, p = 0.005). Positive correlations was noticed between cotinine and both P-selectin ( r = 0.23, p = 0.045) and CXCL4 (r = 0.27, p = 0.02).Conclusion: After smoking cessation in CAD patients following PCI an increase in platelet reactivity and a decrease in P-selectin levels were observed. The risk of thrombotic complications post PCI might be paradoxically enhanced among patients who stopped smoking.