Certain individuals suffer complications after diarrheic episodes caus
ed by Campylobacter, Salmonella, Shigella, or Yersinia. One of these c
omplications may be a sterile arthritis such as reactive arthritis, Re
iter's syndrome and anklylosing spondylitis. Reactive arthritis and Re
iter's syndrome have been shown to have a bacterial etiology; however,
such etiology, while suspected, has not been definitely confirmed for
ankylosing spondylitis. These arthritides show a strong familial asso
cation and here, reactive arthritis, Reiter's syndrome and ankylosing
spondylitis are characterized and the relationship of the diseases to
the major histocompatility complex antigen, HLA-B27, is described. Stu
dies have indicated that there may be molecular mimicry between certai
n microbial antigens and HLA-B27. Molecular mimicry is discussed in te
rms of its possible explanation of the etiology of the reactive arthri
tides. The roles of antibodies, lymphocytes, bacterial antigens, and s
tress proteins in the symptomology of the arthritides are reviewed.