A series of 681 surgically removed appendices were examined for spirochetes. Five hundred seventy-four appendices were removed because of suspected acute appendicitis; the rest were removed per occasionem. One hundred six of the former were histologically normal, whereas 421 showed acute appendicitis. The remaining 47 specimens showed a variety of other pathological conditions, for example, tumors and diverticula. Spirochetes were found in 13 (12.3%) of the appendices removed from patients clinically suspected to suffer from acute appendicitis, but whose appendices were otherwise histologically normal (pseudoappendicitis). Only 3 patients (0.7%) with histologically confirmed acute appendicitis (p less than 0.05) did show spirochetes in their appendices. Of the 107 patients who had their appendices removed per occasionem, 2 patients (1.9%) had spirochetosis (p less than 0.05). The ultrastructure of the spirochetes obtained from appendices with spirochetosis was studied by means of negative staining and ultrathin sectioning. The morphology of these spirochetes was very similar to that of Brachyspira aalborgi, a spirochete recently isolated from rectal biopsy specimens obtained from patients with colorectal spirochetosis.