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.