The second series antigen W27 has been found to have an increased frequency in ankylosing spondylitis (82 % of 17 patients) and juvenile rheumatoid arthritis (Still's disease) (29 % of 24 patients). Three family studies are presented, including one in which the antigen W27 occurs both in the absence and the presence of ankylosing spondylitis. It is suggested that this family illustrates that the relationship between W27 and sacro‐iliitis is affected by non‐HL–A genes. The development of clinical disease depends upon the coincidence of several factors in one individual, some of which are genetically controlled by two or more loci which are not necessarily l