Among a variety of acute abdomens, acute torsion of omentum, first reported by Marchett in 1851, is least suspected under the impression of, most commonly, acute appendicitis and then acute cholecystitis, mesenteric thrombosis, ovarian cyst, perforated peptic ulcer, etc. A 52-years-old woman was admitted on May 2, 1987 with anorexia, nausea and RLQ pain for 2 days. Physical examination revealed tenderness, guarding and rigidity over RLQ. White cell count was 12.100/mm3. A reducible hernia was found in the right inguinal region. The operation through McBurney's incision showed blood-stained fluid. Appendix was slightly congested. A solid, gangrenous mass was palpated at right iliac fossa that disclosed a completely tight torsion of omentum twisting 6 times counterclockwise with distal infarction. Segmental omentectomy, appendectomy and hernioplasty were done. The patient's recovery was uneventful. This case emphasizes the necessity of routine examination of the omentum during the course of abdominal exploration especially when serosanguinous fluid was encountered in the peritoneal cavity.