BACKGROUND Completely isolated enteric duplication cysts (CIDCs) are rare malformations that can occur at any site in the gastrointestinal system. This report describes a woman with a CIDC and an incidental appendiceal neuroendocrine tumor (ANET). CASE REPORT A 26-year-old woman who presented with dysmenorrhea was assessed by ultrasound (US), which revealed a pelvic mass. Other imaging modalities, including magnetic resonance imaging (MRI), failed to clarify the origin of the mass. Intraoperative findings during diagnostic laparoscopy revealed an isolated, ovaloid mass with autonomous peristalsis and a short pedicle towards the root of the ileal mesentery. In addition, the appendix appeared enlarged with a hardened consistency. The mass was resected and an appendectomy performed laparoscopically. The pelvic mass was diagnosed as a CIDC and the appendix was incidentally found to contain a pT3Nx carcinoid tumor. Based on histological examination and guidelines of the European Neuroendocrine Tumor Network (ENET), the patient later underwent a laparoscopic right hemicolectomy. CONCLUSIONS CIDC in adulthood is very rare, especially when combined with an incidentally discovered pT3Nx appendiceal carcinoid tumor. Neither US nor MRI was able to provide a precise preoperative diagnosis. Diagnostic laparoscopy clarified the nature of the mass and revealed a lesion missed during the preoperative workup. Because of the diagnosis of ANET, the patient subsequently underwent a laparoscopic right hemicolectomy.