Colonoscopic diagnosis of colonic polyps is a rather easy task, although some small polyps might be missed when they are situated around the steep angulation or behind the fold. A correct diagnosis of mucosal carcinoma is difficult not only by colonoscopy but also by biopsy because the cancer focus may be small or not exposed on the polyp surface. Therefore, colonoscopic polypectomy is the best procedure of choice for correct histologic diagnosis of mucosal and invasive carcinoma. Only by polypectomy can correct diagnosis of invasive carcinoma be made, although there are various endoscopic signs such as irregularity of the surface configuration and white spots on the surrounding mucosa suggesting invasive carcinoma. Over the course of several years experience of colonoscopy we have encountered small flat elevations under 1 cm in diameter of which 58% were benign adenomas and 42% contained minute foci of carcinoma. These small flat lesions might have been missed quite easily during routine examination in the past and they might play an important role in the pathogenesis of colonic carcinoma. Benign polyp and mucosal carcinoma can be adequately treated by polypectomy only, whereas invasive carcinoma needs further treatment as it has a risk of node metastasis. Risk factors influencing metastasis are lymphatic permeation, poorly differentiated carcinoma, massive invasion close to the cut end. Only when one of these findings is seen in the removed polyp, further surgical treatment must be considered. However, the patient's general condition, age and site of the polyp should be carefully taken into account in order to choose the appropriate treatment in each individual case as the risk of metastasis under such situations mentioned above is not so high. Management of invasive carcinoma is still a controversial issue and a long-term, clinicopathologic study will be needed to solve this question.