<jats:title>Summary</jats:title><jats:p>Bing‐Neel syndrome (BNS), a rare neurological syndrome associated with Waldenström macroglobulinaemia (WM), is a direct involvement of the central nervous system by lymphoplasmacytoid cells characterized with an adverse prognostic. The <jats:italic>MYD88</jats:italic> L265P mutation has been identified in the vast majority of patients with WM. The diagnosis of BNS is often challenging because of the variety of clinical presentations associated with difficult histological techniques. We hypothesized that identification of <jats:italic>MYD88</jats:italic> L265P mutation in the cerebrospinal fluid (CSF) would contribute to the diagnosis of BNS in addition to imaging, flow cytometry and cytology. We identified <jats:italic>MYD88</jats:italic> L265P mutation in the CSF and the bone marrow of all cases of BNS using quantitative polymerase chain reaction q<jats:styled-content style="fixed-case">PCR</jats:styled-content> and Sanger sequencing. Copy neutral loss of heterozygosity including <jats:italic>MYD88</jats:italic> was observed in one case. No mutation of <jats:italic>CXCR4</jats:italic>,<jats:italic> CD79A</jats:italic> and <jats:italic>CD79B</jats:italic> was observed in parallel. We further showed that monitoring the quantitative expression of <jats:italic>MYD88</jats:italic> L265P mutation might be a useful molecular tool to monitor response to chemotherapy using q<jats:styled-content style="fixed-case">PCR</jats:styled-content>. In conclusion, identification of <jats:italic>MYD88</jats:italic> L265P mutation might be a new molecular‐based biomarker tool to add to the diagnostic and monitoring armamentarium for BNS.</jats:p>