Background: A systematic plaque instability is suggested in patients with acute coronary syndrome. Plaque inflammation could be assessed by 18F-fluorodeoxyglucose Positron Emission Tomography (18F-FDG PET). We investigated whether carotid plaque inflammation could be related to coronary plaque instability using 18F-FDG PET. Methods: In 30 (male 14, 48.1짹7.7 yrs) patients who were newly diagnosed as acute coronary syndrome (10 patients, male 6, 46.8짹7.9 yrs) or stable angina (20 patients, male 13, 49.5짹9.8), the co-registration of PET and contrast enhanced computed tomography (CT) images was performed within 1 week after percutaneous coronary intervention. Regional (neck) PET/CT imaging at 1 hour (early scan) and additional scan at 2 hours (delayed scan) after 555 MBq of 18F-FDG injection and the multislice CT angiogram were acquired at 180 min on the Philips GEMINI TF scanner with 16 slice CT. The maximum standardized uptake values (SUVs) were measured in individual plaques. Results: In all patients, carotid plaque with increased 18F-FDG uptake was observed in the fused PET/CT images. Age and gender–adjusted SUV of FDG was significantly higher in patients with acute coronary syndrome than patients with stable angina (mean 4.13짹1.24 (3.19 to 5.27) vs. 2.87짹0.98 (2.47 to 3.62), p<0.01). There were no differences of risk factors between two groups. Conclusions: The patients presenting with acute coronary syndrome demonstrate simultaneous increase of inflammatory activity of the carotid plaque, supporting a potential causal role of inflammation regarding widespread plaque destabilization associated with acute coronary syndrome.