PURPOSE: Biopsy is the gold standard for the diagnosis of primary CNS lymphoma (PCNSL). However, surgical biopsy has problems of morbidity related to hemorrhagic complications and false-negative findings, so safer and more reliable diagnostic methods are required. The aim of this study is to detect the MYD88 mutation, an important driver mutation, in the cerebrospinal fluid (CSF) of patients with CNS lymphoma. PATIENTS AND METHODS: Twenty-six patients with CNS lymphoma (20 primary CNS lymphoma and six CNS relapse from systemic lymphoma) were studied. We extracted cell-free DNA (cfDNA) from CSF by lumbar puncture. cfDNA was extracted from 1 mL of CSF, and Sanger sequencing and droplet digital polymerase chain reaction (ddPCR) were performed. Furthermore, we performed DNA sequencing of MYD88 in 21 cases with available surgically obtained formalin-fixed paraffin-embedded (FFPE) tissue and compared the results. RESULTS: The median cfDNA amount extracted from 1 mL CSF was 219 ng/mL (25th to 75th percentile, 129 to 333 ng/mL). MYD88 mutations were detected from CSF in 76.9% (20 of 26 cases), and L265P in exon 5 was the most frequent mutation in 19 out of 20 (95.0%) cases. S219C in exon 3 was detected in one case. In four patients, MYD88 mutation was confirmed by ddPCR but not by Sanger sequencing. In all 21 cases with sufficient FFPE tissue for DNA analysis, the detection of MYD88 mutation from cfDNA was consistent with those of tumor-derived DNA from FFPE tissue. CONCLUSION: This pilot study provided evidence that the somatic driver mutation MYD88 can be reliably detected by combination of Sanger sequencing and ddPCR in the cfDNA taken from 1 mL of CSF in patients with CNS lymphomas.