In April, 1978, we carried out a survey covering 27 hospitals, in which colonoscopy is performed on a routine basis. With respect to the size of the hospital, the equipment available and the level of training of the examiner, this selection may be regarded as representative. A total of 35,892 colonoscopies, 7,365 polypectomies, 58 electrocoagulations and 14 rugectomies were analysed with respect to the nature, localization and treatment of any complications that arose. The rate of complications seen in diagnostic colonoscopy was 0.008% for bleeding and 0.14% for perforation, the mortality rate being 0.02%. As expected, the complication rate for colonoscopic polypectomy was higher. Bleedings were reported in 2.24%, perforations in 0.34% and deaths in 0.1% of the examinations. Of particular importance would seem to be the possibilities of preventing complications. It was shown on the basis of the survey that a good, standardized training of the endoscopist, the strict observance of the contraindications, the non-use of analgesics and general anaesthesia, fluoroscopic control of "difficult" colons and the use of the best instruments and aids presently available, reduce the complication rate to a minimum.