In order to estimate the incidence and significance of aspiration during anaesthesia, a study of cases in which this complication had occurred was made at the Karolinska Hospital. With the aid of the anaesthetic recordkeeping system of the Department of Anaesthesia and the computer-based register of diagnoses of in-patients at the hospital, all cases in which aspiration was recorded were retrieved. Eighty-three cases of aspiration were retrieved from the file of anaesthetic records and four from the in-patient register. This constitutes an incidence of 4.7 aspirations in 10 000 anaesthetics, or 1 in 2131. The patients most often affected were children and the elderly. In 83% of the cases there were one or more preoperative factors indicating an increased risk for aspiration, such as emergency operation (38 cases, 43%), upper abdominal or emergency abdominal surgery (14 cases, 16%), a history indicating delayed gastric emptying (e.g. peptic ulcer/gastritis, pregnancy, obesity, unusual stress or pain, elevated intracranial pressure, 54 cases, 61%). In 29 cases (33%) there was a history indicating an increased risk of regurgitation, e.g. nasogastric tube, oesophageal disease or pregnancy. In 15 cases of elective surgery, no history of increased risk for aspiration could be found. In 67% of those cases the aspiration was preceded by difficulties involving the airways or intubation. The incidence of aspiration was more than sixfold higher during the night than during regular daytime working hours. In 41 cases (47%) the aspiration led to aspiration pneumonitis confirmed by x-ray. Fifteen patients (17%) needed mechanical ventilation, and four died.(ABSTRACT TRUNCATED AT 250 WORDS)