A histologic examination was made of all appendices delivered to the Dept. of Pathology, Sundby Hospital, Copenhagen, in 1980-87, to ascertain the incidence of Enterobius vermicularis (pinworm) and the possible relation to acute appendicitis. Altogether 2267 appendices were examined, and E. vermicularis was identified in 4.1%. The highest incidence occurred in the 6- to 15-year age group. Infestation was more frequent in female than in male appendices, with the ratio of 1 male to 1.8 females, except in the 11- to 15-year age group, in which the ratio was 1. There was a highly significant difference in the incidence of E. vermicularis in normal appendices and in inflamed appendices, which may indicate that 1) the presence of E. vermicularis in the appendix can give the symptoms of acute appendicitis, or 2) E. vermicularis leaves or does not enter an inflamed appendix.