Thirty-one men and 32 women with ankylosing spondylitis (AS) were evaluated to determine if there are notable differences between the sexes in the nature and course of this disease. The average age at onset was 22 years in women and 23 years in men. Eighteen men, but only eight women, had persistent spinal disease. Conversely, 19 women and only 10 men had clinical evidence of persistent extraspinal involvement. Four women and three men had definite radiographic evidence of articular erosions in the small joints of the appendicular skeleton. Although previous reports have emphasized that women are more likely than men to have AS of the cervical spine and sacroiliac joints, with sparing of the intervening segments, we found this pattern in four men and three women. Involvement of the appendicular skeleton appears to be more common in women and is more apparent clinically than radiographically.