A 38-year-old man presented to our hospital with abdominal pain and melena. Gastrointestinal endoscopy revealed a large gastric ulcer, and the pathological diagnosis of diffuse large B-cell lymphoma (DLBCL) was made based on immunohistochemical findings. Left diplopia developed soon after commencement of chemotherapy. Despite normal findings from head MRI, orbital involvement in DLBCL was detected with 18F-fluorodeoxyglucose positron emission tomography (FDG-PET). The patient was treated with salvage chemotherapy with success. Treatment analysis using FDG-PET for patients with DLBCL, especially for those with clinical symptoms and negative findings on conventional modalities, may be useful for assessing disease status and adjusting treatments.