Rheumatoid arthritis of children occurring in three major subtypes defined by the symptoms present during the first six months following onset: systemic-onset (Still's Disease, Juvenile-Onset), polyarticular-onset, and pauciarticular-onset. Adult-onset cases of Still's disease (still's disease, adult-onset) are also known. Only one subtype of juvenile rheumatoid arthritis (polyarticular-onset, rheumatoid factor-positive) clinically resembles adult rheumatoid arthritis and is considered its childhood equivalent. [MeSH]