A 21-year-old male with developmental delay presented with abdominal pain of two days' duration. He was afebrile and his abdomen was soft with mild diffuse tenderness. There were no peritoneal signs. Plain x-ray demonstrated a large air-filled structure in the right upper quadrant. Computed tomography of the abdomen revealed a 9 x 8 cm structure adjacent to the hepatic flexure containing an air-fluid level. It did not contain oral contrast and had no apparent communication with the colon. At operation, the cystic lesion was identified as a duplication cyst of the sigmoid colon that was adherent to the right upper quadrant. The cyst was excised with a segment of the sigmoid colon and a stapled colo-colostomy was performed. Recovery was uneventful. Final pathology was consistent with a duplication cyst of the sigmoid colon. The cyst was attached to the colon but did not communicate with the lumen.