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Of twenty-one patients with juvenile chronic arthritis (seventeen with juvenile rheumatoid arthritis and four with juvenile ankylosing spondylitis) who had total hip replacement before the age of thirty years, thirteen patients (62 per cent) showed different degrees of ectopic ossification: nine (53 per cent) of the seventeen with juvenile rheumatoid arthritis and all four of those with juvenile ankylosing spondylitis. Human leukocyte antigen B5 was found with an increased frequency in the patients with ectopic ossification. Functional capacity was not impaired regardless of the severity of the ectopic ossification. As in patients with adult-onset disease, ectopic ossification after total hip replacement occurred with a higher frequency in the patients with juvenile-onset ankylosing spondylitis than in those with juvenile-onset rheumatoid arthritis.