A 45-year-old woman who had undergone surgical repair for an atrial septal defect at the age of 7 years presented with multiple anterior mediastinal masses. Positron-emission tomography (PET) with 2-[18F]fluoro-2-deoxy-d-glucose (FDG) revealed high uptake in the masses. The findings were interpreted as mediastinal malignant tumors with dissemination. Biopsy was performed, and the histological diagnosis was phosphoglyceride crystal deposition disease without neoplastic changes. Although phosphoglyceride crystal deposition disease is rare, it should be recognized as a potential interpretive pitfall that mimics a malignant tumor in FDG-PET findings in a patient after cardiac surgery.