Lyme disease and tick-borne encephalitis have similar principal epidemiological features chiefly associated with their basic carrier and determinants of human infection. The cases of Lyme disease, as evidenced by laboratory studies, indicate that there are active natural foci of this infection in the city, frequently combining with natural foci of tick-borne encephalitis. At the same time a great quantity of cases of tick-borne encephalitis and Lyme disease (mixed infection) is notified in Tomsk.