Previous research has established the relationship between viral infection with adenovirus, influenza B virus and Coxsackie B virus, and mesenteric adenitis. A prospective controlled survey of the incidence of viral infection in mesenteric adenitis was undertaken in children attending a children's hospital over a period of 13 months. Eighteen patients, together with 18 controls who were matched for age, sex, and date of presentation, were studied. There were five cases of adenovirus type 7 infection in the mesenteric adenitis group and none in the control group (P=0.023--Fisher's exact test). The clinical problem at presentation is in differentiating mesenteric adenitis from appendicitis. If further work shows that viral infection is indeed more common in mesenteric adenitis, and is limited to a narrow range of viruses, then rapid identification of these viruses in children with "acute abdomens" would be a valuable aid to differential diagnosis.