A 51-year-old woman was admitted to our hospital because of fever and pain in the right buttock. She had ulcerative colitis. She was given a diagnosis of sacroiliitis complicated with ulcerative colitis, based on a physical examination and magnetic resonance imaging (MRI) . Her sacroiliitis was successfully treated by leukocytapheresis (LCAP) . Sacroiliitis complicated with ulcerative colitis is rare in Japan, and its treatment and pathogenesis remain unclear. We report an unusual case of sacroiliitis complicated with ulcerative colitis, which was successfully treated by LCAP.