The observed frequency of apparent homozygotes for HLA--B27 (15.5%) was significantly greater than the estimated expected frequency (4.2%) among 58 B27-positive Caucasian patients with ankylosing spondylitis (AS) (P less than 0.0005). Search of the literature uncovered four other studies in each of which the frequency of apparent homozygotes was shown by our analysis to be greater than expected. These analyses indicated that B27 homozygotes are more susceptible to developing AS than are B27 heterozygotes. Comparison of the clinical features of AS showed no differences between heterozygotes and apparent homozygotes except for a higher frequency of involvement of peripheral joints in the latter group.