A 67-year-old man with a biopsy proving prostate diffuse large B-cell lymphoma was referred for an F-FDG PET/CT scan for postchemotherapy treatment monitoring. The FDG PET/CT scan revealed a significant reduction of prostate lymphoma and a prostatorectal fistula formation. The patient developed Klebsiella pneumoniae sepsis, and a follow-up FDG PET/CT 3 months later revealed multifocal lung, bone, and muscle infection. The FDG PET/CT scans of this patient not only demonstrated the effectiveness of chemotherapy for prostate lymphoma but also the complications of a prostatorectal fistula and multiple infectious foci.