Primary osteosarcoma of the spine not associated with any predisposing factor is rare. The authors describe a case of osteosarcoma of the lumbar spine in a 44-year-old woman with biopsy-proven ulcerative colitis. The patient underwent bone scanning, computed tomography (CT) and magnetic resonance imaging (MRI), and the authors present the first record of the MRI appearance of this disease in a vertebral body. Relative to classical osteosarcoma of long bone, this case involved a higher degree of soft-tissue ossification, which was readily visible with both CT and MRI. The patient had extensive metastasis, including metastasis to the perirenal fat, and died 10 months after presentation.