The efficacy of screening colonoscopy in general use remains to be determined. Here we report data over a 39-month study period collected in a nationwide online registry. Data from consecutive screening colonoscopies performed on asymptomatic patients in the practices of 280 participating gastroenterologists (age 55-99 years) were collected in an online registry. The number and histology of colorectal polyps and carcinomas, complication rates of colonoscopy and polypectomy were registered. Advanced adenoma was defined as an adenoma of >or= 10 mm in diameter, villous or tubulovillous in histology, or presence of high-grade dysplasia. A total of 269 144 colonoscopies (male 44%) were evaluated. Tubular, villous/tubulovillous adenomas and invasive cancers were found in 15.6, 3.7, and 0.8%, respectively. Advanced adenomas amounted to 7.1%. In 95% of polyps greater than 5mm and less than 30 mm immediate polypectomy was performed. In 399 of the 575 carcinomas with complete tumor node metastasis stages, which were detected during colonoscopy, early stages dominated (UICC stages I and II in 43 and 27%, respectively). Complication rate was low and no fatalities were observed: cardiopulmonary complication in 0.10% of the colonoscopies, bleeding in 0.8% of polypectomies most of which were managed endoscopically (surgery in 0.03% of polypectomies). Perforation occurred in 0.02% of the colonoscopies and 0.09% of polypectomies. Colonic neoplasias are detected in about 20% of patients most of which are immediately removed by polypectomy at a low risk. Polypectomy of adenomas and low UICC stages in cancer patients during screening colonoscopy may be tools for fighting colorectal cancer mortality.