Ankylosing spondylitis and related spondyloarthropathies form a family of rheumatic diseases that are characterized by inflammatory peripheral and axial arthritis, with predilection for sacroiliitis, and a remarkably strong association with a genetic marker, HLA-B27. The association with B27 has provided a great impetus to the epidemiologic studies of spondyloarthropathies and also helped broaden the clinical spectrum of these diseases. There are at least six subtypes of B27, and the x-ray crystallographic structure of the common B27 subtype (B*2705) is now known. Endogenous peptides bound in the B27 antigen-binding cleft have been isolated and all seem to be nonamers (i.e., nine amino acids long).