In a prospective study from 1988 to 1991, we discovered 31 flat colon adenomas in 29 patients and performed endoscopic resection of 27 of these lesions, using the strip biopsy technique. All of these lesions were safely and completely resected even in the thin walls of the colon. All 27 lesions were histologically flat adenomas confined to the mucosa, with a maximum diameter of 3-20 mm. Thirteen were identified as severe dysplasia, three as moderate dysplasia, and 11 as mild dysplasia. These lesions were more clearly defined after they were sprayed with indigo carmine endoscopically. The 13 flat adenomas with severe dysplasia were morphologically slightly depressed red lesions with elevation of the surrounding mucosa (six lesions) and flat round red lesions with no changes in the surrounding mucosa (seven lesions). Due to the high incidence of severe dysplasia found in flat adenomas, removal using strip biopsy is recommended.