Ankylosing spondylitis is three times less common in American blacks than in whites. It is extremely rare in African blacks of unmixed ancestry. A histocompatibility antigen HLA-B27, which does not exist in African blacks of unmixed ancestry, and is present in eight percent of white and two to four percent of the American black population, is strongly associated with ankylosing spondylitis and Reiter's disease. B27 is present in more than 80 percent of white patients with ankylosing spondylitis or Reiter's disease but in less than 60 percent of American black patients. Other genetic and environmental factors may be of major importance in the genesis of these diseases in American blacks. For diagnostic purposes the absence of B27 is of less importance in excluding these diseases in blacks than in whites.