The authors have attempted to evaluate the effectiveness of abdominal ultrasonography in the diagnosis of an omental tumor. Our subject was a 76 year-old man with leiomyosarcoma of the omentum. In ultrasonographic examination, an irregularly shaped heterogeneous large tumor exsisted between the stomach, spleen and pancreas. The tumor displaced the greater curvature of the stomach, but no obvious invasion to the stomach was detected. The boundary between the tumor and spleen was indistinct as was that between the tumor and pancreas, suggesting tumor invasion to both the spleen and pancreas. CT findings were consistent with those of the ultrasonography. The upper gastrointestinal series revealed a displacement of the greater curvature of the stomach, with partial rigidity of the wall. Barium enema indicated downward displacement of the transverse colon. Preoperative diagnosis was a solid mass located between the greater curvature of the stomach and the transverse colon, with invasion to the spleen and the pancreas. The tumor was considered to have originated in the omentum. Pathological studies revealed this tumor to be a leiomyosarcoma of the omentum that had invaded the spleen and pancreas.