Lyme boreliosis is a multiorgan disease, transmitted by ticks, which is caused by the bacteria Borrelia burgdorferi. The increase of morbidity is recorded and the area of occurrence includes nowadays all territory of Poland. The clinical picture is very differentiated. Symptoms depend on the duration of illness and the level of involvement of different systems. The disease may affect the skin, joints, nervous system as well as the heart. Information about epidemiology, pathogenesis and virulence of Borrelia burgdorferi were presented. The authors disscused characteristic manifestations and methods of identification. Recommended antibioticotherapy was also taken into consideration, depending on the patient’s age and the stage of illness. In the article a 11-yearold boy’s case was presented. The child was admitted to hospital due to arthralgia, a characteristic circular rash and an episode of high fever. He had a history of a tick bite 5 weeks earlier. He did not travel outside Warsaw. Basic tests were performed and they did not show any abnormalities. Disseminated Lyme borreliosis with arthritis was diagnosed. Ceftriaxon was applied and resolution of symptoms was achieved. Later in the result for Borrelia burgdorferi infection a high level of IgM was indentified. After admitting full treatment a decreasing titer of antybodies was observed and the patient did not present any abnormal signs.