Enterobius vermicularis is one of the common parasites in the world. The infestation rate of Enterobius vermicularis is higher in children than in adults. Moreover, the egg-positive rate of Enterobius vermicularis is higher in the mentally retarded population. We report the case of a ten-year-old, mentally retarded boy, who was diagnosed as having acute appendicitis associated with Enterobius vermicularis.