The role of obstruction in the pathogenesis of acute appendicitis was studied by measuring the pressure in the appendix perioperatively in 24 patients operated on with an appendectomy because of suspected acute appendicitis. The technique used involved inserting a fine needle through the apex into the appendix lumen and measuring the hydrostatic pressure required to inject saline solution. In three patients with a gangrenous appendix found at operation, and in two patients with a phlegmonous appendix, there were signs of obstruction of the appendix lumen as revealed by increased pressure within it. In 14 patients with a phlegmonous appendix found at operation, no signs of obstruction were found. These experimental data suggest that obstruction is not an important causative agent of acute appendicitis, but might develop as a result of the inflammatory process.