The Wels General Hospital is a large teaching hospital where all endoscopies are performed by the staff of the First Department of Internal Medicine. The reports of 10,000 colonoscopies between January 1st, 1986 and February 15th, 1994 were scrutinized by the authors, 15 complications of diagnostic and therapeutic colonoscopies were identified. There were seven perforations and one ileus associated with diagnostic colonoscopy (8989 patients) and five bleeding episodes and two perforations after polypectomy (1609 polyps removed; 1011 patients). Four bleeding episodes after polypectomy were controlled endoscopically with circumlesional injection of adrenaline. One patient required emergency surgical treatment. All perforations were treated surgically. Two elderly patients with multimorbidity died after perforation and emergency surgery (peritonitis and pulmonary embolism being the respective causes of death). The frequency of perforation was 0.09% (9 of 10,000), the mortality rate was 0.02% (2 of 10,000), and the bleeding rate after polypectomy was 0.05% (5 of 10,000). The frequency of complications in relation to the experience of the physicians revealed that five of seven perforations associated with diagnostic colonoscopy occurred in the hands of less experienced endoscopists (doctors in training with less than 400 completed endoscopies). Life-threatening complications related to the performance of flexible fiberoptic colonoscopies are rare, especially during diagnostic procedures. As reported in the literature, our results show that the risks are greatest with relatively inexperienced physicians but are not entirely avoidable even with the most experienced endoscopists.