Background: Multiple sclerosis (MS) is an inflammatory demyelinating disease of central nervous system with secondary progressive neurodegenerative changes lead to accumulation of disability. The important role of mitochondrial dysfunction has been reported in the pathogenesis of MS. Objective: To investigate the serum lactate level in MS and to explore the prognostic value of lactate regarding disability and progression Methods: This case-control study was conducted on 80 Egyptian subjects; 40 MS patients and 40 normal healthy individuals. Patients were subjected to detailed history taking, thorough neurological examination and clinical assessment of the disability using Expanded Disability Status Scale (EDSS(.Serum level of lactate was measured in both groups. Results: In comparison to control, serum lactate level was significantly higher among MS patients. There was no significant correlation between serum lactate level and any clinical variants apart from age and age of onset of MS. Conclusion: Measurement of serum lactate may be helpful in MS and this support the hypothesis of the critical role of mitochondrial dysfunction and axonal damage in MS. Our study recommend future researches on large cohort of MS patients to assess long term prognosis in patients with elevated lactate levels .In addition , we recommend measurement of lactate in cerebrospinal fluid of MS patients to evaluate its clinical significance as a biomarker in MS.