Twenty-three (85.2%) of 27 female patients with ankylosing spondylitis were positive for HLA-B27 antigen, including 15 of 18 white women (83.3%) and eight of nine black women (88.9%). Ankylosing spondylitis in women appears to have the same high association with HLA-B27 antigen as that reported in predominantly male studies (88% to 96%). Determination of this antigen may be useful in the evaluation of female patients suspected of having ankylosing spondylitis.