Neurolymphomatosis (NL) is the neoplastic endoneurial invasion of the peripheral nervous system. It is a rare and challenging diagnosis, but it should be taken into account in the differential diagnosis of peripheral neuropathy, particularly in patients with a documented history of haematologic malignancy. Magnetic resonance (MR) neurography is very useful in diagnosis and especially when correlated with positron emission tomography/computed tomography (PET/CT). In equivocal cases, nerve biopsy can be considered when the benefit outweighs the risk. We aimed to report a case of a 30-year-old male patient who was in complete remission from T-lymphoblastic lymphoma, presenting with clinical findings indicating initially ulnar entrapment. However, with the demonstration of brachial plexopathy with axonal loss in electrodiagnostic studies, MR imaging neurography dedicated to brachial plexus was carried out and revealed pathological enhancement associated with mild fluorodeoxyglucose (FDG) uptake on PET/CT. Moreover, NL, due to the relapse of T-cell lymphoma, was diagnosed through incisional biopsy, showing diffuse infiltration of blast cells positive for terminal deoxynucleotidyl transferase, CD3 and CD10. Further, radiotherapy and systemic chemotherapy were initiated, and symptoms recovered with regression of pathological FDG uptake.