A 53-year-old male with no past medical history presented with abdominal pain for 2 days, accompanied with fever and poor appetite. Leukoerythroblastosis along with elevated lactate dehydrogenase and a bone marrow exam led to the diagnosis of diffuse large B-cell lymphoma, germinal center B-cell (GCB) type. After the 5th cycle of rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisolone, progressively decreased visual acuity along with left ptosis occurred. Drug-related optic neuropathy was suspected, and initial magnetic resonance imaging (MRI) and cerebral spinal fluid (CSF) studies revealed negative results. However, meningeal lymphomatosis with bilateral optic nerve infiltration was finally diagnosed by subsequent MRI and CSF studies after 1 month. This rare case highlights the pitfalls of CSF studies. Based on previous studies, an adequate sample (>10.5 ml), rapid processing within 1 h, and serial testing at least twice can improve the rate of positive results while reducing the false-negative rate. We hope that this case can remind clinicians of the possible diagnosis of lymphomatous optic nerve infiltration from systemic lymphoma, and that properly conducted CSF studies can help to avoid missing this diagnosis.