Paraneoplastic neurological syndromes are independent of metastasis, direct tumour infiltration, or known indirect mechanisms such as toxicity, ectopic hormone secretion, or induced coagulopathies.1 Paraneoplastic neurological syndromes may affect any part of the nervous system, and are believed to result when an immunologic response is directed against shared antigens that are ectopically expressed by the tumour, but otherwise predominantly expressed by the nervous system (onconeural antigens).1 Antibodies can be detected in the serum or cerebrospinal fluid of many, but not all, patients with PNS.2 Diagnosing PNS requires identification of the type of neurological syndrome based on neurological signs and symptoms, the detection of well-characterised onconeural antibodies, and the presence of a cancer.3 Paraneoplastic neurological syndromes are rare in patients with solid tumours, and probably even rarer among patients with lymphomas.4 The predominant types of PNS in lymphomas are paraneoplastic cerebellar degeneration in Hodgkin's lymphoma, and dermatomyositis/polymyositis in both Hodgkin's lymphoma and non-Hodgkin's lymphoma. Traditionally considered a variant of multiple sclerosis, presence of the disease-specific aquaporin-4 antibody, which plays a direct role in the pathogenesis of NMOSD, distinguishes the two entities.5 Recently, NMOSD is increasingly recognised as a paraneoplastic disorder especially in men or in patients who present in older age.6 Paraneoplastic NMOSD has been reported in a wide variety of tumour histological types but mostly in solid tumours.6 We recently encountered a definite case of PNS in a patient who was diagnosed with mantle cell lymphoma (MCL) and shortly afterwards developed neurological symptoms due to NMOSD. Early recognition of a neurological syndrome as paraneoplastic often leads to the discovery and treatment of the underlying tumour, which is a crucial step in the management of the PNS.1 2 Author contributions All authors have made substantial contributions to the concept or design; acquisition of data; analysis or interpretation of data; drafting of the article; and critical revision for important intellectual content.