We report a case of steroid-induced osteonecrosis in a patient with refractory ulcerative colitis. A 31 year-old woman presented suffering from refractory ulcerative colitis. She had been treated by prednisolone for ten years. Sharp pain and swelling appeared suddenly in her right knee. Conventional radiography revealed neither osteoporosis nor a fracture. However, magnetic resonance imaging by T1-, T2 and T2*-weighted images revealed irregular heterogeneous areas of low- and high-intensity in her right femur and tibia. For a precise early diagnosis of osteonecrosis, bone magnetic resonance imaging was found to be an excellent diagnostic tool.