Ixodes tick-borne borrelioses (TBB) are a group of etiologically independent acute infectious natural-focal transmissive and polysystemic diseases caused by Borrelia and transmitted by the ticks. These diseases tend to be chronic and recurrent, as well as latent, mainly involving the skin, nervous system, locomotor apparatus and heart. The signs of nervous system lesion are recorded in both acute (subacute) and chronic course of the disease. The comparatively low notification rate of TBB in the Ulyanovsk Region, particularly involving the nervous system, in the absence of the gold marker of this disease--erythema migrans (it was absent in both cases) did not allow clinicians to make a correct diagnosis at the early stages of patient examination. The currently available methods, in this case, Western blot, are of great importance in diagnosing the disease. By taking into account delayed antibody generation within the first weeks of the disease, it is necessary to continue examination by the methods available in the practical health arsenal (ELISA) and in the later periods, with the epidemiological history and clinical picture of encephalitis being borne in mind.