PURPOSE: To determine the characteristic CT findings of infectious sacroiliitis and ankylosing spondylitis. MATERIAL AND METHOD: We retrospectively reviewd CT findings in 10 patients with infectious sacroiliitis confirmed by culture and clinical follow ups and in 5 patients with ankylosing spondylitis by HLA-B27 typing. Mean ages were 30 years in ankylosing spondylitis and 29 years in infectious sacroiliits. CT scans were obtained with GE 9800 or Toshiba 900-S scanner. We analyzed CT findings in regard to the morphology and the degree of bone erosion, and the adjacent soft tissue change. RESULT: All cases of ankylosing spondylitis had bilateral and asymmetic bone erosion, predominantly in ilium, showing subchondral sclerosis on ilium. Infectious sacroiliitis showed unilateral involvement and soft tissue swelling in 10 cases and abscess in 5 cases. CONCLUSION: We concluded that CT was useful in the differentiation between infectious sacroiliitis and ankylosing spondylitis.