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Lyme arthritis (LA) may be confused with other rheumatic diseases, particularly in the absence of a history of erythema migrans (EM). We report the case of a 12-year-old patient who developed a large effusion of the right knee joint. The titer for antinuclear antibodies was 1:80 and the test for rheumatoid factor was negative. Investigations for antibody response to Borrelia burgdorferi demonstrated remarkable elevation of IgG antibody and no specific IgM response. These results were confirmed by immunoblotting reactivity with the bands p83/100, p58, p43, p41, p39, OspA, p30, OspC, p21, and p17. We subsequently learned that the child had suffered a tick bite followed by an EM 5 years earlier and had been treated with trimethoprim/sulfamethoxazole at that time. The patient now was given intravenous ceftriaxone, 2 g daily for 14 days. In the absence of clinical improvement 3 weeks later a knee joint aspiration was performed which resulted in a positive polymerase chain reaction (PCR) test for B. burgdorferi DNA (OspA) in the synovial fluid. The patient fully recovered 2 months later without further treatment. The case indicates that the latency period between EM and onset of LA may last up to 5 years. In addition to serologic test methods, analysis of synovial fluid using PCR may be decisive for making the final diagnosis of LA.