The prevalence and symptoms of pinworm infection were determined in a shanty town in Lima, Peru. In 206 families, pinworm infection rates were highest in primary school age children (42%). Approximately one-fourth of pre-school children and secondary school-age children were infected with pinworms. Two examinations detected 74% of patients who were positive after 5 examinations. Symptoms often attributed to pinworm infection, such as perianal itching enuresis, and teeth grinding, occurred in a similar proportion of infected children (15%, 17%, 13%, respectively) and non-infected subjects (11%, 13%, 11%, respectively). Enuresis was more common in primary school-age children with high pinworm egg counts than in their non-infected contemporaries. 52% ( 28 54) of children under 5 years old became reinfected within 6 months of effective treatment. In a community where water is scarce and hand washing is infrequent, the high rate of perianal itching is probably an important source of faecal-oral contamination. The low morbidity and high reinfection rate make routine treatment of pinworm infection in third world countries a low priority, except when clinically indicated. © 1991.