PURPOSE: Intra-abdominal cystic masses originating from the retroperitoneum, mesentery or omentum are very rare and mostly benign tumors, but sometimes present as a complicated cyst encasing the major organs. METHODS: We analyzed the clinical findings, histologic diagnosis, and surgical outcomes in children who underwent operation for retroperitoneal, omental, and mesenteric cyst from 1998 to 2010, retrospectively. RESULTS: Twenty-three patients (male, 12; female, 11) underwent the operation at a median age of 46 months (range, 9 days to 16 years). Among them, 17 cysts presented one or two symptoms such as abdominal mass, abdominal pain or abdominal distension. The median duration of symptoms was 7 days (range, 1 day to 365 days). Five were detected prenatally. Ten cysts were found in retroperitoneum, 8 in the omentum and 5 in the mesentery. The median diameter was 13 cm (range, 3 to 30 cm). Twenty cysts were completely removed. Five mesenteric cysts required bowel resection and anastomosis. Three of retroperitoneal cysts were impossible to complete excise because of location and extensiveness. Pathologically, 20 cysts were lymphangioma and 3 were pseudocyst. The morbidity was one of adhesive ileus and the mortality was one who had extensive retroperitoneal cyst with mesenteric cyst. He died from sepsis. During follow-up period, there was no recurrence. CONCLUSION: Preoperative diagnosis and localization for these cysts are very difficult. Complete excision was possible in almost all cases despite the size, bringing a favorable outcome. The possibility of this disease entity should be considered as the cause of acute abdomen.