Eighty-six patients who had adenoma and underwent colonoscopic polypectomy were examined by colonoscopy over a long period (mean follow-up time: 5 years) to establish an effective long-term surveillance program. The incidence of neogenetic lesions in patients with a large monoadenoma (diameter 0.5 cm; LA 69.7%) and polyadenoma (PA 74.2%) was higher than in those with a small monoadenoma (diameter <0.5 cm; SA 27.3%). We found 4 and 2 carcinomas in patients with LA and PA, respectively. The high incidence of neogenetic lesions in patients with LA (40.7%) and PA (50.0%) did not decrease with the passage of time. Long-term surveillance with total colonoscopy should be performed every two years in patients with large monoadenoma or polyadenoma for effective detection of neogenetic lesions.