The volume and pH of the gastric contents aspirated prior to anaesthesia were measured in 640 acute surgical patients. The material comprises 100 patients aged between 0 and 15 years, 415 between 16 and 60 years, and 125 between 61 and 93 years. The possible relation between the gastric contents and the patients' sex, age, weight, and surgical disease (abdominal or orthopaedic) was examined. Relations to fasting time, composition of meal (solid/liquid), the consumption of alcohol, the use of tobacco, the injection of analgesics, gastritis/ulcus anamnesis, nausea and pains were also calculated. The volume of gastric contents was higher in women than in men, and the number of potential risk patients among women was higher (p less than 0.02). However the difference is not considered to be of any clinical importance. In relation to body weight children have a relatively high volume of gastric contents, and these are more acid in children and young people than in grownups. If we delimit potential risk patients by means of the volume and pH of the gastric contents, there are for these reasons more risk patients among young than among elderly people. The groups with orthopaedic and abdominal diseases had the same number of potential risk patients, but the patients in the former group were younger than in the latter. Analgesics do not increase the number of risk patients; however, the number was higher among patients having pains (p less than 0.05). The consumption of alcohol did increase the number of risk patients (p less than 0.001). The length of fasting time also considerable influenced the volume of gastric contents in the acute surgical patient.