A 77-year-old man who had undergone mitral valve replacement 5 years previously presented with an intrapericardial mass. Computed tomography and magnetic resonance imaging showed that the mass lesion contained hematoma components. Positron-emission tomography (PET) with 2-[¹⁸F]fluoro-2-deoxy-D-glucose (FDG) revealed uptake in the peripheral rim of the mass. These findings suggested the presence of hematoma associated with a malignant lesion. Surgical resection was performed, and the histological diagnosis was chronic expanding intrapericardial hematoma without neoplastic changes. Chronic expanding intrapericardial hematoma is a rare disease but should be considered when an expanding mass is found in a patient after cardiac surgery. The FDG-PET findings of chronic expanding hematomas, including FDG uptake in the peripheral rim of the mass as a result of inflammation, should be recognized as a potential interpretive pitfall that mimics a malignant tumor.