By serum typing, it is indicated that HLA B27 may be associated with the susceptibility to ankylosing spondylitis (AS). We analyzed DNA restriction fragment length polymorphism (RFLP) in 28 Chinese AS patients and 99 healthy controls, using a 1.4 Kb HLA class I cDNA probe. The results showed that the frequencies of the 8.1 Kb EcoRI, 5.2 Kb EcoRI and 21.9 Kb XbaI fragments were found to be significantly increased in affected patients (P less than 0.0001, P = 0.0015, respectively), but that of 19.2 Kb XbaI fragment was decreased (P = 0.0021). The data suggest that AS may be a polygenic disease; furthermore, B27 and 8.1 Kb EcoRI band may be two different factors responsible for the susceptibility or just in linkage disequilibrium with the susceptible gene(s).