A 23-year-old female in the 28th week of pregnancy was admitted with an acute abdominal distension. The plain X-ray films showed a marked gaseous dilatation of the colon from the cecum to the splenic flexure. A tube colostomy was performed. Her postoperative course was uneventful and she improved progressively. Her pregnancy and delivery elapsed uneventfully there after. After the delivery she again complained of marked abdominal distension. The colonoscopic decompression in combination with the administration of Cisapride was successfully accomplished. Ogilvie's syndrome is characterized by an unobstructed colon which progresses to marked dilatation. When the colonic diameter becomes 12cm or more, surgical decompression should be recommended to avoid the hazard of spontaneous rupture. This is the first reported case of Ogilvie's syndrome in pregnancy in Japan.