We investigated 30 consecutive Brazilian patients with definite ankylosing spondylitis (AS) fulfilling the New York and the European spondyloarthropathy study group classification criteria. The mean age at study was 37 years old and the mean disease duration was 17 years. Bone densitometry employed the dual-energy X-ray absorptiometry (DEXA) technique, using a Hologic QDR-1000/W densitometer. Axial bone mineral density (BMD) was measured in the lumbar spine (L1-L4) and appendicular BMD was measured in the total proximal femur and sub-regions (neck, greater trochanter, intertrochanter and Ward's triangle). Based on World Health Organisation criteria, the lumbar spine showed osteopenia or osteoporosis in 50% of the patients, while 86% had osteopenia or osteoporosis in the total proximal femur. When compared with the normal population, the patients showed a significant BMD decrease in the lumbar spine and total proximal femur with sub-regions, except for the femoral neck. A comparison of BMD between patients with active and inactive disease did not reveal a significant effect of clinical disease activity on the lumbar spine and total proximal femur with sub-regions, except for Ward's triangle. Concerning disease chronicity, there were significant positive correlations between disease duration and lumbar spine, total proximal femur, greater trochanter and intertrochanteric regional BMD. This false increase in lumbar spine BMD found mostly in patients with long standing AS was due to the presence of paravertebral calcification and ossification. We conclude that the bone mass loss in AS is better evaluated in the proximal femur, because of the greater sensitivity of bone densitometry in this region, which is almost free of artefacts.