The clinical manifestations, magnetic resonance imaging (MRI) data, blood and cerebrospinal fluid (CSF) tests were analyzed in 19 patients with early-onset (before the age of 17 years) multiple sclerosis (MS) and in 100 patients with the definite diagnosis of MS with adult onset. Between group differences in sex ratio and initial neurological symptoms, with the prevalence of acute brainstem dysfunction (37%) and optic neuritis (37%) in the patients with early onset MS, were revealed. In childhood, the disease course was poorer with early disablement on the background of attendant unfavorable conditions. A sensitivity of various MRI parameters was analyzed and the essential proportion of the tumor-like acute inflammatory lesions was shown in early onset MS (29%). Immunological tests (albumin coefficient evaluation) revealed that in early onset MS hematoencephalic barrier dysfunction was registered more often, comparing to adult onset MS (100% vs 50%). The peculiarities of biases in concentrations of kappa-free and lambda-free light immunoglobulin chains in blood and CSF were detected. Higher levels of the kappa-free light chains concentrations in CSF were mostly found in the patients both with early and adult onset MS (86% and 99%, respectively) that confirmed the diagnostic value of this index. The results obtained may improve MS diagnosis in children.