We examined the distribution of non-B27 alleles of the HLA-B locus among B27+ patients with ankylosing spondylitis (AS), to detect any additional HLA-B locus allele(s) that may act in conjunction with B27 to increase susceptibility to AS. HLA-Bw60 (or B40 when the Bw60,61 split of B40 was not typed for) was shown to be increased among B27+ AS patients in each of 5 independent data sets. This increase was statistically significant in 4 of the 5 data sets studied, and the overall significance was P less than 0.00001. Susceptibility to AS in B27+ individuals was further increased by a factor of approximately 3 when Bw60 was also present. The distribution of HLA-A alleles on the B27-bearing haplotypes in AS patients was not significantly different from that in normal controls. On the other hand, the distribution of HLA-A alleles on Bw60-bearing haplotypes was significantly different from the distribution of A alleles on Bw60 haplotypes in the general population (P less than 0.0005). Bw60 was not increased in B27- patients with AS. A dominant mode of inheritance generally fits AS; however, our sib pair analysis indicates that the B27,Bw60 disease subgroup follows a more recessive mode of inheritance.