A case of primary torsion of the omentum is reported. A 37-year-old male was admitted with the chief complaint of RLQ pain of 3 day's duration on June 10, 1986. Tenderness and Blumberg's sign were found at Mc Burney's point with leucocytosis of 11,500/mm3. His body temperature was not elevated. Operation was carried out under the diagnosis of suspicion of acute appendicitis. Laparotomy revealed a dark-red omental mass of 12 X 5 X 1 cm which had been formed by torsion (7 times, clock-wise) and a little clear ascites. The omental mass was resected, and appendectomy was also performed. The postoperative course was uneventful. Eighteen cases including this case in the Japanese literature were reviewed and reported.