A seroconversion to Borrelia burgdorferi, the etiological agent of
Lyme disease, was documented in a 5 year old girl from central Ontario. Our patient developed juvenile arthritis in January, 1984 when she was 14 months old. It was relatively well controlled with aspirin. Her arthritis flared in April, 1986 at which time seroconversion to B. burgdorferi was demonstrated by indirect immunofluorescence and by enzyme linked immunosorbent assay. She was treated with phenoxymethyl penicillin and has had no further flare of her arthritis. The possible relationship between her rheumatic
disease and her seroconversion to B. burgdorferi is discussed.