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A 61-year-old man was diagnosed with advanced-stage diffuse large B-cell lymphoma with systemic and cerebrospinal fluid (CSF) involvement. MR scans of the neuro-axis showed no central nervous system or nerve root abnormalities, but CSF testing identified lymphoma cells on cytology/flow cytometry, confirming nervous system involvement. After three out of six planned cycles of intrathecal methotrexate plus rituximab, cyclophosphamide, doxorubicin, vincristine and prednisone, he had achieved a complete metabolic response by positron-emission tomography (PET) scan. Despite this transient apparent systemic response and before his final cycle, he developed a subacute, rapidly progressive, painful sensorimotor polyradiculoneuropathy with associated bilateral facial, left oculomotor neuropathies and neuromuscular respiratory weakness within a period of 3 weeks. He required intubation and mechanical ventilation and received intravenous immunoglobulin 2 g/kg over 5 days for an initial diagnosis of Guillain–Barré syndrome. Electromyography and nerve conduction studies showed bilateral facial neuropathies and a mixed axonal and demyelinating polyradiculoneuropathy. He had minimal improvement and an MR scan of the lumbosacral plexus identified right sciatic nerve enlargement, T2 hyperintensity and enhancement; a PET-CT showed mild ¹⁸F-fluorodeoxyglucose (FDG) uptake only at the sciatic nerves, right greater than left (figure 1). A targeted right sciatic fascicular nerve biopsy confirmed neurolymphomatosis (figure 2). This progressed (figure 1) despite escalation of treatment to high-dose intra-venous methotrexate, rituximab and temozolomide and he died within 1 month.