Five families whose members had 2 distinct B27 haplotypes, and in which numerous members had ankylosing spondylitis (AS), were studied to see whether, in each family, both B27 haplotypes were associated with similar risks for the development of AS. In 4 of the 5 families, all family members with AS (4, 2, 3, and 2 individuals, respectively) shared the same B27 haplotype; however, 1 was homozygous for HLA-B27 and carried both haplotypes. By statistical analysis, we showed that the B27 haplotype carried by the eldest member with AS in each family was more likely to be associated with disease. If additional genetic factors play a role in the development of AS, these preliminary findings suggest that they are probably located around the major histocompatibility complex.