In terms of adult-onset definitions, rheumatoid arthritis (RA) and ankylosing spondylitis (AS) are rarely diagnosed in children. Adult RA is in most aspects similar to seropositive polyarticular arthritis in children, but AS differs in its clinical presentation according to age at onset. In general, the nomenclature and classifications of arthritis in children encompass subgroups with specific signs or laboratory tests and pathogenic mechanisms that distinguish one clinical form from the other. While one of these subgroups corresponds to RA, the one related to AS usually includes children with undifferentiated SpA and not definite AS. Thus, comparisons of RA and AS in children actually correspond to comparisons of various forms of childhood arthritis, currently classified as juvenile idiopathic arthritis (JIA) and AS in its early undifferentiated form. In this paper, we review these to finally compare the two populations.