Among 86 patients selected as possibly having ankylosing spondylitis because of clinical symptoms and radiologically normal sacroiliac joints. HLA-B27 was positive in 41%. Four years later a representative sample of 38 individuals were re-examined and radiographed. HLA-B27 positive patients developed sacroiliitis as defined by radiological criteria twice as often (P less than 0.05). They also showed increased uptake of technetium 99 m upon quantitative scintigraphy with a region of interest method and more often probable or definite ankylosing spondylitis as defined by the New York criteria. Further differences between the HLA-B27 positive and negative follow-up groups concerned the frequency of clinical symptoms and peripheral arthritis. It is suggested that HLA-B27 typing may be helpful both in diagnosis and in judging the prognosis of possible or abortive ankylosing spondylitis.