Seronegative Lyme arthritis.

We present a 10-year-old girl who had been diagnosed with juvenile idiopathic arthritis 5 years before and who experienced a flare of arthritis affecting one knee while she was off medication for almost 3 years. Seronegative Lyme arthritis had to be diagnosed based on the detection of Borrelia burgdorferi DNA in synovial fluid. No humoral immune response to Borrelia burgdorferi was detectable before, at the time of diagnosis and up to 3 years later.