In a prospective evaluation of acute intestinal obstruction in emergency surgery, 3550 consecutive patients were studied. In the vast majority of patients (75%), obstruction was due to the external hernia, the inguinal hernia being by far the commonest type. However, the ascaris worm in children, volvulus of the sigmoid colon in adults, and intussusception in both children and adults, were significant causes of the disorder, and together accounted for 18% of the patients. Obstruction by the ascaris worm is easy to diagnose (by stool microscopy), and effective treatment (with antihelminthics) is readily available and cheap. A large number (90%) of the volvulus patients required resection for gangrene of the colon, thus arguing a strong case in support of laparotomy and inspection of the colon whenever feasible. A significant (41%) proportion of intussusception cases were adult, and in 33% of this group the lesion was associated with a tumour of the small bowel. The chief reason for death (10%) was late reporting to hospital.