Sixty patients with multiple sclerosis and 12 healthy controls were examined. The fatigue was measured by the Fatigue Severity Scale (FSS). Concentration of platelet serotonin was analyzed with immunoassay techniques («Serotonin ELISA"). The results of the study have shown the primary role of fatigue in multiple sclerosis, with the prevalence of this syndrome reached 57.6%. The patients with multiple sclerosis had low concentration of platelet serotonin that was correlated with clinical features of the disease.