The frequency of B27 homozygosity was determined in 100 patients with B27 positive ankylosing spondylitis (AS). Six patients appeared to be B27 homozygous. The difference between the observed frequency of homozygosity of 6% and the expected frequency of 2.3% (from results on 725 blood donors) was not significant (p greater than 0.05). All 6 patients had moderate to severe disease. Subsequently, the families of 10 patients with AS were studied. The families were selected because 2 distinct B27 haplotypes were present. No difference was observed between the prevalence of AS in B27 homozygous or heterozygous family members. Although B27 homozygosity may have some influence on the severity of the disease, it does not increase the risk for AS even in families of patients with AS.