Fibrodysplasia ossificans progressiva is a rare heritable disorder of connective tissue characterized by skeletal malformations and by progressive heterotopic ossification. It has been suggested that the genetic marker human leukocyte antigen B27 may be associated with fibrodysplasia ossificans progressiva, as it is with ankylosing spondylitis, another disorder with less severe hyperostosis. Genomic deoxyribonucleic acid from 23 classically affected patients with fibrodysplasia ossificans progressiva was screened for the human leukocyte antigen B27 allele by polymerase chain reaction. Only two of the 23 patients (9%) with fibrodysplasia ossificans progressiva who were examined showed the presence of the human leukocyte antigen B27 allele, an incidence that corresponds to the 8% frequency of individuals within the general population not affected with ankylosing spondylitis. These data suggest that the human leukocyte antigen B27 allele does not occur more commonly in the genotype of patients with fibrodysplasia ossificans progressiva than in the general population, and that the pathogenesis of heterotopic bone in fibrodysplasia ossificans progressiva differs from that of ankylosing spondylitis and other human leukocyte antigen B27 positive disorders.