The 60-kD heat shock protein (hsp60) has been implicated in the etiology and pathogenesis of both experimental and naturally occurring autoimmune diseases such as juvenile chronic arthritis (JCA). Human hsp60 is expressed in inflamed synovial tissue, and T lymphocytes both from peripheral blood and synovial fluid show reactivity to human hsp60. Because the anti-hsp60 B lymphocyte response has been less well studied, we have determined the occurrence of IgG anti-human hsp60 antibodies in patients with JCA and various other autoimmune diseases of childhood. Serum IgG anti-human hsp60 antibodies in JCA patients were significantly higher compared with control children (358 and 163 U/mL, respectively). Within the group of JCA patients, the highest antibody titers were found in the subgroup with a polyarticular onset of JCA. IgG anti-human hsp60 antibody levels in synovial fluid were 3- to 4-fold higher compared with paired serum samples. Because this difference was not found for total IgG or for irrelevant antibodies (anti-polyribosylribitol phosphate), this suggests local anti-hsp60 antibody production in the synovial compartment. The occurrence of anti-hsp60 antibodies is not specific for JCA but also is found in children with systemic lupus erythematosus and in cystic fibrosis, whereas mixed connective tissue disease and insulin-dependent diabetes are negative in this respect. Whether the anti-human hsp60 antibodies are directed toward species-specific sequences or to conserved sequences of the hsp60 molecule remains to be determined.