To compare the bone mineral density (BMD) and determine the frequency of osteoporosis in mild and advanced ankylosing spondylitis (AS) cases. Seventy three patients with AS were enrolled in this study. The BMD was analyzed at the lumbar spine and hip by dual energy X-ray absorptiometry. The patients were diagnosed as being "normal, osteopenia, or osteoporosis" according to the WHO classification. Using the BASRI-lumbar and BASRI-hip scores, the patients were grouped in mild and advanced AS categories. The mean BMD in the lumbar spine and hip of patients with mild and advanced AS was similar (p > 0.05). While 61.6% of the patients were found to have osteopenia or osteoporosis in the lumbar spine, 46.6% had osteopenia or osteoporosis in the total hip. Of the patients with advanced AS 54.3% had osteopenia or osteoporosis in the lumbar spine, 75% in the total hip. Of the patients with mild AS patients had 68.4% osteopenia or osteoporosis in the lumbar spine, and 42.3% in the total hip. The osteopenia or osteoporosis frequency of the mild and advanced cases of AS in the lumbar spine was similar (p > 0.05). In the advanced AS patients, osteopenia or osteoporosis frequency was significantly higher in the total hip than in the mild AS patients (p < 0.05). In conclusion, there was evidence of osteoporosis in both the advanced AS and mild AS patients. The reason why the anteroposterior lumbar DXA results in the advanced AS patients were similar to the mild ones may be due to the existence of syndesmophytes and ligament calcification. In these cases, it is more convenient to use a hip DXA for assessing the extent of osteoporosis.