In childhood, the histocompatibility antigen HLA-B27 is associated with juvenile ankylosing spondylitis, Reiter's syndrome, and the sacroiliitis of juvenile psoriatic arthritis, and inflammatory bowel diseases. The low frequency of signs or symptoms of spine or sacroiliac joint disease at onset differentiate these disorders from their adult counterparts and make early diagnosis difficult. The presence of enthesitis and the absence of rheumatoid factor and antinuclear antibody suggests the diagnosis of one of the seronegative spondyloarthropathies. The mechanisms by which HLA-B27 positivity confers susceptibility to disease are uncertain. To date, no genetically determined differences in the HLA-B27 molecule, or in the T cell receptor which recognizes antigen in the context of HLA-B27, have been consistently shown to be associated with the presence of disease. There is no explanation for the onset of disease in childhood in some individuals.