The paper presents the results of a study of 176 cases of mixed infection of tick-borne encephalitis and Ixodes tick-borne borreliosis in children in the Kuznetsk coal fields. The mixed infection presented as meningeal, meningoencephalitic, and feverish forms. Mixed infection is characterized by a severe course and it is not attended by the symptoms that are characteristic for monoinfections, which requires the immunological verification of their diagnosis. Unlike tick-borne encephalitis, the meningeal and meningoencephalitic forms of the mixed infection are marked by the great magnitude and duration of a temperature reaction and overall cerebral and focal manifestations. The outcome is characterized by the formation of symptoms of chronic tick-borne encephalitis in 25% of the cases of the meningoencephalitic form of the mixed infection.