We investigated the frequency and distribution of osteopenia according to the clinical severity in ankylosing spondylitis (AS). Bone mass was measured in men with mild (n = 45) and severe AS (n = 31) with dual-energy X-ray absorptiometry (DEXA). Definition of clinical severity was based on the Schober's test. Osteopenia was commonly detected (48% in mild AS and 39% severe AS) and, in mild disease, more frequently observed at the lumbar spine than any of the proximal femur sites. In severe AS, however, the frequency of osteopenia at the femoral neck and Ward's triangle was as high as at the lumbar spine. Both bone mineral density and T-scores in severe disease were lower than in mild disease at the femur neck, Ward's triangle, and total proximal femur, but not in the lumbar spine. The progression of osteopenia may be reflected more reliably at proximal femur sites than at the lumbar spine.