INTRODUCTION: This study investigates the significance of glial fibrillary acidic protein (GFAP) and ubiquitin C-terminal hydrolase L1 (UCHL-1) in cerebrospinal fluid (CSF) of patients with multiple sclerosis (MS) and peripheral neuropathy (PN). METHODS: We included 41 MS patients, 35 PN patients, and 36 controls across 5 sites. MS patient data included lesion counts, disease activity, albumin quotient, and Expanded Disability Status Scale (EDSS) scores. PN patients included those with acute and chronic inflammatory demyelinating polyneuropathy and sensorimotor neuropathy based on nerve conduction studies. CSF concentrations of GFAP and UCHL-1 were measured using the MILLIPLEX Map Human Neuroscience Magnetic Bead Panel 1. RESULTS: Both GFAP and UCHL-1 levels were significantly higher in the two patient groups compared to controls. In the MS group, GFAP showed a strong correlation with disease duration, EDSS score, non-enhancing lesions, and the CSF/blood albumin quotient. UCHL-1 levels were significantly higher in patients with active disease (gadolinium-enhancing lesions). The combination of UCHL-1 and GFAP improved diagnostic accuracy (AUC 0.895, 95% CI 0.780-1.000) compared to the independent measurement of either marker for indicating Gd-negative lesions. In the PN group, CSF GFAP levels were significantly lower in patients with purely demyelinating neuropathy compared to those with axonal or mixed neuropathy. CONCLUSION: GFAP serves as a sensitive marker for axonal damage in PN, while UCHL-1 closely correlates with disease activity in MS patients.