In a family study, sacroiliitis was found in 15 (25%) out of 61 HLA-B27 positive first-degree relatives (FDR) of 20 randomly chosen HLA-B27 positive ankylosing spondylitis (AS) patients and never in 41 HLA-B27 negative FDR. Eight (53%) of the 15 FDR with sacroiliitis fulfilled the New York criteria for AS. In the population study the prevalence of HLA-B27 positive AS was found to be 0.1%. AS will develop in 1.3% of all HLA-B27 positive individuals belonging to the general population.