We describe herein the case of a 10-year-old boy in whom generalized peritonitis was caused by perforation of a tubular communicating ileal duplication cyst. Alimentary tract duplication cysts are rare congenital malformations, found primarily in children under the age of 15 years. The perforation was caused by heterotopic gastric mucosa within the duplication, giving rise to peptic ulceration in the adjacent intestinal mucosa. The presence of heterotopic gastric tissue is a primary cause of perforation that has been reported in as many as one third of patients with small intestine duplications; however, the detection of such ectopic tissue is time-consuming, and there is no readily available method of diagnosing tubular duplications. Although very few patients present with peritonitis as the initial manifestation, the possibility should be borne in mind when diagnosing and planning therapy for such emergencies, particularly in children.