We describe a prospective analysis, in one hospital, of reported significant observations involving unsafe practices and working conditions during anaesthesia. Of the 549 significant observations reported voluntarily during a period of 18 months, 82% involved occurrences which were considered preventable and 27% could have been fatal if they had not been recognized and corrected. Ninety-three percent of incidents did not lead to a negative outcome. Human error was responsible for 411 (75%) reports. Lack of vigilance and failure to check were the most frequently reported factors associated with human error. Significant observations involving errors in drugs administration were the most frequent. Forty-five percent of all reported significant observations were made during maintenance of anaesthesia.