To explore the significance of sacroiliac joint aerocele in the diagnosis of ankylosing spondylitis, the data of 196 patients with ankylosing spondylitis (AS) were collected during December of 2008 to May of 2009. And 50 patients with osteoarthritis (OA), 15 patients with sclerosing osteitis (SO) and 47 patients with sacroiliac joint tumors were investigated as the control groups. The feature of sacroiliac joint aerocele in computed tomography (CT) images was observed carefully. In AS group there were 130 patients (66.3%) diagnosed as AS according to CT results, and 32 of them (24.6%) were observed with aerocele within sacroiliac joint cavity, majority of whom were earlier AS patients with slight bone destruction. Other 66 patients were diagnosed as early AS according to magnetic resonance imaging (MRI) and ultrasonography. CT examination showed that the 66 patients did not have apparent bone destruction, of whom, 26 (39.4%) patients had aerocele within sacroiliac joint cavity. Among the control groups of 15 (15/50, 30.0%) patients with OA, 5 (5/15, 33.3%) patients with SO were observed sacroiliac joint aerocele. The 47 patients with sacroiliac joint tumors were observed with bone or cartilage destruction, but without signs of sacroiliac joint aerocele. The sacroiliac joint aerocele in CT images of AS patients usually appeared as spots, streaks, small or larger round blocks, and it often happened in patients with earlier stage of diseases. Sacroiliac joint aerocele may be useful to early diagnosis of AS. To explore the significance of sacroiliac joint aerocele in the diagnosis of ankylosing spondylitis, the data of 196 patients with ankylosing spondylitis (AS) were collected during December of 2008 to May of 2009. And 50 patients with osteoarthritis (OA), 15 patients with sclerosing The feature of sacroiliac joint aerocele in computed tomography (CT) images was observed carefully. In AS group there were 130 patients (66.3%) diagnosed as AS according to CT results, and 32 of them (24.6%) were observed with aerocele within sacroiliac joint cavity, majority of whom were earlier AS patients with slight bone destruction. Other 66 patients were diagnosed as early AS according to magnetic resonance imaging (MRI) and ultrasonography. CT examination showed that 66 patients did not have apparent bone destruction, of whom, 26 (39.4%) patients had aerocele within sacroiliac joint cavity. Among t The 47 patients with sacroiliac joint tumors were observed with bone or cartilage destruction, 15 (15/50, 30.0%) patients with OA, 5 (5/15, 33.3%) patients with SO were observed sacroiliac joint tumors but without signs of sacroiliac joint aerocele. The sacroiliac joint aerocele in CT images of AS patients usually appeared as spots, streaks, small or larger round blocks, and it often happened in patients with earlier stages of diseases. Sacroiliac joint aerocele may be useful to early diagnosis of AS.