Previously called juvenile rheumatoid arthritis and juvenile chronic arthritis, this is a set of related conditions of unknown cause affecting children. Characteristically, the synovial membrane of a joint or joints becomes inflamed and swollen for at least six weeks (and often for much longer even years). About 1 in 10,000 children develop it each year, many of whom have certain HLA genetic markers, thought to be important in determining who gets the illness. Inflammatory cytokines play an important part.