Lyme borreliosis in the Sverdlovsk region was epidemiologically analysed on the basis of the patients' referral to the Center of Lyme Borreliosis. From 1991 to 1993 a total of 873 cases of specified Lyme borreliosis were recorded. The main epidemiological characteristics of Lyme borreliosis coincided with those of tick-borne viral encephalitis. The endemic areas virtually covered all the districts of the region with the foci concentrated around large cities in the south and south-west of the region. The risk of Lyme borreliosis was found to increase with the patient's age and the duration of tick's attachment. Rural and urban residents are at an equal risk of the disease.