Introduction. In the available literature, we could not find any description on the combined pathology consisting of a volumetric formation in the form of a dermoid cyst and malformation of the abdominal cavity in the form of an isolated duplication of small intestine with localization in the retroperitoneal space. Purpose. To present a curative and diagnostic algorithm for a rare pathology in the retroperitoneal space in a 15-year-old child. Material. A 15-year-old girl who was operated on for a dermoid cyst and an isolated doubling of small intestine localized in the retroperitoneal space and having no clinical manifestations. Results. When performing the spiral computed tomography (SKT) of the patient’s abdomen because of polytrauma, a cystic mass localized in the left half of the abdomen was accidentally revealed. Due to the critical state of the girl, it was decided to observe the child’s state and to make a decision on surgical treatment after her recovery. Later, at the preoperative thorough examination (ultrasound, CT, MRI), a retroperitoneal formation (dermoid cyst?) was suspected, and a part of small intestine isolated from the gastrointestinal tract but adjacent to this formation was visualized. Laparoscopy confirmed the diagnosis, and both formations were successfully removed from the retroperitoneal space. conclusion. Modern imaging techniques have the potential to make accurate diagnostics in rare surgical diseases before surgery, to determine surgical tactics and minimize surgical aggression due to minimally invasive technologies.