A 74-year-old man presented to the hospital complaining of weight loss, increasing fatigue, and blurred vision. The abdominal ultrasonography initially revealed a massive lesion in the mesentery, which was later confirmed by a contrast-enhanced CT scan. The 18F-FDG PET/CT scan showed a single, solitary hypermetabolic mass. The patient was finally diagnosed with Waldenström macroglobulinemia with mesentery involvement by the histopathological examination.