Lyme disease is a multisystem infection affecting all age groups. In this study an attempt was made to determine whether the patient's age influences the course of the disease. One hundred and fifty patients with diagnosed Lyme disease were included in the study. Two serological methods were used to detect antibodies to Borrelia burgdorferi and to confirm the diagnosis: an indirect immunofluorescence assay (the Russian strain Ip-21) and Western blot. The course of Lyme disease did not differ from that seen in Europe and North America. However, a few clinical differences between groups were observed. In the first age group (0-15 years) the most common manifestation was flu-like symptoms with fever. Neither radiculoneuritis nor polyneuropathy was observed in this age group. Late manifestations were rare and the outcome of the disease was benign. The course of the disease in the second group (16-40 years) was most similar to that in childhood and the also outcome was similar. Carditis and erythema multiple were significantly more common in the second group (16-40 years) than in the other age groups. No differences were found between the third (41-60 years) and fourth (> than 60 years) group in the frequency of flu-like symptoms, erythema multiple and aseptic meningitis. However, the most important clinical sign in this group was involvement of the nervous system whereas in the third group this was joint damage. This feature deserves attention because, as a rule, the presence of an articular syndrome determines the prognosis of LD.