PURPOSE: A laterally spreading tumor (LST) has its own characteristic features and growth pattern. Information about LST is scanty in Korea, therefore this study was designed in order to contribute to the literature. METHODS: In this study, 43 patients with LSTs were included. The diagnoses were made by colonoscopy in all cases. Treatment options included endoscopic resection, transanal excision, and surgical resection. In reviewing and analyzing the cases, we made a special emphasis on size, classification, histology, and treatment. RESULTS: The most frequent location was the rectum, followed by the sigmoid colon and the ascending colon in that order. Lesions smaller than 20 mm accounted for 69.8%. Granular homogeneous LSTs, 41.9%. Lesions larger than 20 mm, except granular homogeneous LSTs, showed an abrupt increase in malignancy rate. Tubular adenomas accounted for 65.1%. The overall malignancy rate was 20.9%, and the submucosal cancer rate, 9.3%. There were no malignancies in the granular homogeneous LSTs. The malignancy rate for the mixed-nodule type lesions was 33.3% (4/12), and the nongranular LSTs, 38.5% (5/13). Polypectomy was done in 37.2% of the lesions, endoscopic mucosal resection (EMR) in 16.3%, and endoscopic piecemeal mucosal resection (EPMR) in 16.3%. The overall endoscopic resection rate was 83.7% (36/43). EMR was applicable to lesions smaller than 20 mm, and EPMR to those larger than 20 mm. Transanal resection was done in 2 cases with lesions. Five cases were resected surgically. Four of them were submucosal invasive lesions, and one, a mucosal lesion which was wide and had initially been thought to be a submucosal cancer. There were two recurrences during the average 15-month follow-up period. The follow-up rate was 81.4% (35/43). Of these 2 recurring cases, one patient was treated endoscopically and the other, transanally. CONCLUSIONS: LSTs show different behavior depending on the endoscopic classification. Granular homogeneous LSTs are seldom larger than 30 mm and are good candidates for endoscopic treatment. The mixed-nodule type and the nongranular type show a marked predisposition to malignancy when they are over 20 mm, and nongranular-type LSTs have a higher rate of submucosal invasive cancers. Thus, in the cases of the mixed-nodule and nongranular types, careful consideration should be given for deciding between endoscopic treatment and surgical resection. Complete resection should be assured to prevent recurrence, and follow-up surveillance is required in all lesions for more than 3 to 5 years.