Introduction: Most colorectal polyps are treated endoscopically based on the theory of the adenoma-carcinoma sequence. However, sometimes it is difficult to differentiate between adenomatous and hyperplastic polyps. Unnecessary biopsies or endoscopic resections of non-neoplastic lesions are yet not uncommon. Purpose: The purpose of this study is to evaluate the usefulness of magnifying colonoscopy for differential diagnosis between hyperplastic polyp and adenoma in colorectal concer screening. Objects and Methods: Between January 2001 and December 2001, 376 subjects were enrolled in this study and 538 colorectal lesions were resected endoscopically (hot biopsy) regarded as neoplastic lesions. The term of this study was divided into two periods of 6 months for each. During the first period, colonoscopies were performed only with conventional scopes by two experienced colonoscopists without knowledge of magnifying colonoscopy. During the second period, one colonoscopiet with thorough knowledge of magnifying colonoscopy used either conventional or magnifying colonsocope, at random. The procedures were divided into 3 groups according to the period in which they were performed and the colonoscope used ; group A ; conventional colonoscopy in the first period, group B ; conventional colonoscopy in the second period, and group C ; magnifying colonoscopy in the second period. The percentages of histologically non-neoplastic lesions among the endoscopically resected lesions were compared between these three groups. Results: The percentage of non-neoplastic lesions in group A and group B were 21% and 19% respectively, both of which are significantly higher than that of group C, 7%. Conclusions; Magnifying colonoscopy was considered to be useful in differential diagnosis between hyperplastic polyp and adenoma in coloreetal cancer screening.