Objective The purpose of this study was to investigate myocardial findings of F-18-fluorodeoxyglucose PET/computed tomography (F-18-FDG PET/CT) after thoracic radiotherapy.Methods F-18-Fluorodeoxyglucose PET/CT examination was performed in 38 patients at least 4 months after radiotherapy. Patients with known cardiac diseases, high cardiovascular risk factors, or diabetes mellitus were excluded.Results On visual analysis, 28 patients were seen to have regional myocardial F-18-FDG uptake (74%), five patients were seen to have diffuse myocardial F-18-FDG uptake (13%), and five patients were seen to have no significant myocardial F-18-FDG uptake (13%). Regions of interest were drawn on irradiated and nonirradiated segments of the myocardium. The standardized uptake value measurements of the 28 patients with regional myocardial F-18-FDG uptake revealed significantly higher values in the irradiated segments in comparison with nonirradiated segments (P<0.001).Conclusion Annular or focal increased F-18-FDG uptake in irradiated myocardial segments may be observed after thoracic radiotherapy. These myocardial uptake regions were not consistent with the vascular territory of coronary arteries, and the patients had no prior myocardial infarction or coronary artery disease. The patients with a history of thoracic radiotherapy, who showed increased F-18-FDG uptake on PET/CT, especially in the basal myocardium, should be followed up cautiously for the early diagnosis of cardiac events.