Purpose: Intra-abdominal adhesion related to prior abdominal surgery is the most common cause of small bowel obstruction (SBO). However, there are subsets of patients with SBO without a history of previous operation. We studied the characteristics of these patients. Methods: The medical records of 311 patients underwent operations at Seoul National University Hospital between Jan. 1994 and Oct. 2005 were reviewed. A retrospective analysis of the incidence, etiology, diagnostic method, preoperative hospital stay, operative findings and methods, complication rates, postoperative hospital stay, re-admission rate, and reoperation rate of SBO was performed, and the results were compared with those of patients with a history of previous operation. Results: Among the 311 patients (54.4±14.7 yr, M:F=1.5:1), 48 patients (15.3%) had no history of operation. The etiologies of SBO without a history of operation were malignancy (23.0%), bezoar (14.5%), adhesion (10.4%), Crohn’s disease (10.4%), tuberculosis (8.3%), and appendicitis (8.3%). SBO without prior abdominal surgery showed a longer preoperative hospital stay, which was not statistically significant. The accuracies of CT and small bowel series among patients without a history of operation were 68.4% and 54.5%, respectively. SBO without prior abdominal surgery showed a lower complication rate (8.3% vs. 26.6%, P=0.006) and shorter postoperative hospital stays (12.7±6.9 days vs. 16.1±10.4 days, P=0.032). Conclusions: Among the patients who underwent an operation for SBO, 15.3% had no history of previous operation. The most common cause of SBO without a history of operation was malignancy. SBO without a history of operation showed a lower complication rate and a shorter postoperative hospital stay compared with SBO with a history of operation.