Discordance for ankylosing spondylitis in monozygotic twins.

Three monozygotic twin pairs, each over the age of 45 years, with ankylosing spondylitis are described. In two pairs there is discordance for this disease and in the third pair there is a marked difference in the severity of this disease. These findings provide evidence that environmental factors are necessary not only for the initial development of ankylosing spondylitis, but possibly also for determining its future severity.