Objectives To investigate the diagnostic and prognostic values of cerebrospinal fluid (CSF) CYFRA 21-1 in patients with leptomeningeal carcinomatosis (LMC). Methods Concentration of CSF CYFRA 21-1 was detected using electro-chemiluminescent immunoassay. The difference in level of CYFRA 21-1 between 61 patients with LMC and 200 patients with other neurological disease was evaluated, and diagnostic performance of CSF CYFRA 21-1 was investigated. In LMC patients treated with ventriculo-lumbar perfusion (VLP) chemotherapy, prognostic performance of CSF CYFRA 21-1 was evaluated. Results The CSF CYFRA 21-1 was significantly higher in LMC patients than that in patients with other neurological diseases (p<0.001). The sensitivity, specificity, accuracy, and positive and negative predictive values were 80.3%, 95.0%, 91.6%, 83.1%, and 94.1% for CSF CYFRA 21-1, and 65.6%, 100%, 92.0%, 100%, and 90.5% for CSF cytology, respectively. The use of high CSF CYFRA 21-1 and/or positive CSF cytology findings resulted in an increased sensitivity of 85.3%, without compromising specificity. LMC patients with high CSF CYFRA 21-1 were more frequently accompanied by positive CSF cytology results than those with low CSF CYFRA 21-1. The median overall survival was longer in LMC patients with low CSF CYFRA 21-1 than in those with high CSF CYFRA 21-1 (p=0.031). During VLP chemotherapy, the clinical responses were found to be correlated with the biological responses, including the level of CSF CYFRA 21-1 and intracranial pressure. Conclusions CSF CYFRA 21-1 might be regarded as an additional diagnostic tool for LMC and a potential significant prognostic biomarker in LMC patients treated with VLP chemotherapy.