BACKGROUND The purpose of the present study was to measure the incidence and type of incidents that occurred in relation to anaesthesia and surgery during a 1-year period in six Danish hospitals. Furthermore, we wanted to identify risk factors for incidents, as well as risk factors for incidents being deemed critical. METHODS A four-page questionnaire describing patient data, type of anaesthesia and surgery, and occurrence of incidents was filled in for all anaesthesias in the period, and subsequently processed. The incident reporting form incorporated 59 predefined adverse events. The occurrence of one or more of these events described the incident. When the reporting anaesthetist deemed that an incident had harmed the patient, that incident was defined as critical. RESULTS A total of 64,312 anaesthesias were reported, and in 7754 of them one or more incidents occurred. A total of 8510 incidents occurred, 4077 of them were solely related to the anaesthetic procedure, 3702 described events related to physiological alterations in the patient (physiological incidents). Three hundred and twenty-three of the incidents were deemed critical. High ASA score, high age, abdominal surgery, urgent surgery, and complex anaesthetic procedure were significant risk factors for physiological incidents and critical incidents. We could not identify a simple subset of adverse events that could adequately be used to describe the critical incidents. However, complex incidents, i.e. incidents involving more than one adverse event, were more likely to be deemed critical than simple incidents. CONCLUSION The incidence of incidents was 12.1%, and the incidence of critical incidents was 0.5%. Incidents were more likely to be deemed critical in patients with an ASA score of III and above undergoing urgent surgery.