We report 2 independently-conducted family studies of HLA-B27 positive probands with ankylosing spondylitis (AS), both of which support the view that the clinical spectrum of AS is broader than ordinarily assumed, and should include individuals who have symptomatic disease but who do not show radiologic evidence of abnormalities of the sacroiliac joints or the spine. In the Cleveland study of 100 relatives of 30 B27 positive AS probands, 9 relatives did not show radiologic abnormalities of the sacroiliac joints or the spine but had symptoms of chronic inflammatory back pain previously reported to be characteristic of AS. These 9 relatives were all subsequently found to possess B27, in contrast with only 27 of 60 asymptomatic relatives (P less than 0.01). In the Leiden study of 101 relatives of 20 randomly chosen B27 positive AS probands, 13 of 86 relatives without radiographic evidence of sacroiliitis reported "thoracic pain and stiffness," as defined in the Rome criteria for AS. Twelve of these 13 symptomatic relatives were B27 positive. In contrast, among the remaining 73 relatives, only 33 were B27 positive (P less than 0.01). The occurrence of these characteristic spondylitic symptoms in B27 positive, but not B27 negative, relatives of AS probands suggests that the spectrum of the clinical manifestations of AS may include individuals with symptomatic disease, but without radiographic evidence of abnormalities of the sacroiliac joints or the spine. The relatively large number of females we found in this group suggests that women are more likely to manifest this variety of disease than are men.