Lyme disease is one of the most frequent infections in the Russian Federation. In patients with multiple sclerosis, the Borrelia infection may act as a trigger in some cases and also worsens the clinical course of multiple sclerosis. One hundred patients with definite multiple sclerosis were examined including 19 patients with the combination of multiple sclerosis and Lyme disease. The difficulties of diagnosis, clinical features and treatment of Lyme disease in patients with multiple sclerosis are discussed.