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Infiltration of the peripheral nervous system (PNS) by lymphoma, called neurolymphomatosis, is a rare condition among the spectrum of lymphoma-associated neuropathies; its diagnosis is challenging. Cerebrospinal fluid (CSF) analysis is of great value, but nerve biopsy (NB) may be necessary to prove invasion by malignant cells. Clonality polymerase chain reaction (PCR)-based analysis is a validated method in the diagnosis of hematological malignancies, but there are very little data on its diagnostic yield on NB samples. We explored the contribution of NB with clonality analysis to the diagnosis of neurolymphomatosis in 15 patients with negative CSF analysis. Moreover, we assessed the performance of clonality testing in a case-control manner, using patients with inflammatory infiltrates on NB as controls. Neurolymphomatosis was the first manifestation of lymphoma in 60% and could be diagnosed on routine histology alone in 40%. Clonality testing showed monoclonal rearrangement in 86.7% and was unsuccessful in 8.1%. Performance of clonality testing was as follows: 92.9% positive predictive value, 90% negative predictive value, 86.7% sensitivity, 94.7% specificity. This study confirms the diagnostic challenge of neurolymphomatosis, the usefulness of NB in patients with negative CSF analysis, and highlights the high yield of PCR-based clonality testing to assess the malignant nature of PNS lymphoid infiltrates.