Striking prevalence of ankylosing spondylitis in "healthy" w27 positive males and females.

Ankylosing spondylitis is diagnosed once or twice in each 1000 males and one tenth as frequently in females, but the true prevalence is unknown.
Indentification of genetic marker, HL-A W27, for susceptible persons has provided a tool facilitating epidemiologic studies and allowing identification of " control " populations without the marker.
Evaluation by postal questionnaires, and pelvic radiography of 78 HL-A 27W-positive blood donors selected from a group of apparently healthy subjects revealed 14 who satisfied the criteria for definite ankylosing spondylitis.
The prevalence was similar in both sexes.
One hundred and twenty-six W27-negative controls matched for race, sex, and age failed to yield a single case.
For a person of either sex with HL-A W27, there appears to be about a 20 per cent chance that ankylosing spondylitis will develop, suggesting a prevalence of 10 to 15 per thousand.
Hitherto accepted figures may underestimate the frequency by a factor of 10 to 20.. 
