Adult rheumatoid factor (RF) positive rheumatoid arthritis (RA) and RF positive arthritis of childhood are associated with DRB1*0401 in Caucasians, and DRB1*0405 in Orientals, and in Ashkenazi and nonAshkenazi Jews. Certain other DRB1 alleles (DRB1*0101,1001) which have a similar sequence in the 3rd hypervariable region of the 1st domain are also associated with RA, but they appear to function as weaker risk factors. The difference in the relative strength of the associations is likely to be due to structural differences in the 1st and 2nd variable regions of the first domain of these alleles. Similarities and differences in the HLA associations between North American Caucasoid patients with juvenile arthritis in Dallas, TX, USA, and in Prague, Czechoslovakia, are discussed.