Starting from index patients with confirmed Reiter's disease, a clinical and immunogenetic study was performed on 12 families in which there were further cases of arthritis. Altogether 51 family members were investigated and some information was available on 15 additional members. In most families there were two or three affected members in addition to the proband. The manifestations included acute polyarthritis (16 cases), which frequently followed urethritis or occurred as a complication of Yersinia or Shigella infection, and chronic arthritis (9 cases), either ankylosing spondylitis or peripheral arthritis. The latter characteristically had a remitting course, affecting mainly the large joints. Not a single subject had sero-positive rheumatoid arthritis. The HLA B27 gene was detected in all 12 families, and served as the main indicator of the familial trait for developing arthritis. In individual patients however, the association was not especially close, since there were members with this antigen who did not have arthritis in spite of a seemingly adequate triggering stimulus and others who had arthritis but not the antigen.