Objective. To analyze clinical, neuroimaging and laboratory characteristics of acute disseminated encephalomyelitis (ODEM) in adults with an analysis of the frequency of diagnostic errors at the initial examination stage. Material and methods. The study included 23 patients hospitalized with a diagnosis of ODEM. The analysis of clinical characteristics, MRI results and cerebrospinal fluid (CSF) was performed. The diagnosis of the disease in each case was specified after a full examination and dynamic observation. Results. The diagnosis of ODEM was confirmed only in 16 cases. In 6 cases, the final diagnosis of multiple sclerosis (MS) was made, and one case of Susak syndrome, primary CNS lymphoma and levamisole-associated multifocal inflammatory leukoencephalopathy was also detected. It has been shown that ODEM in adults is characterized by the acute development of multifocal brain lesions in combination with clinical manifestations of encephalopathy, which in half of cases is preceded by an infectious disease. There were no specific clinical, neuroimaging and laboratory features, which could allow differentiation of ODEM from onset of MS. Conclusion. To make a correct diagnosis in the first episode of acute multifocal brain lesion, a dynamic observation, including repeated MRI, is needed. The development of specific biomarkers may be of great importance for the early differential diagnosis of demyelinating diseases.