Internal herniation, defined as a protrusion of the bowel through a peritoneal or mesenteric aperture into a compartment of the abdominal cavity, is a potentially serious complication following Roux-en-Y gastric bypass. In view of the high risk of strangulation and volvulus with possible progression to necrotic bowel, sepsis and multiple organ failure, early diagnosis and prompt surgical intervention are crucial to minimise morbidity and mortality rates. As little is known about internal herniation, pathogenesis, consequences, clinical presentation and diagnostic approach are discussed in this article. A review of the literature and the consideration of three cases make it clear that diagnosis is often delayed due to a vague, non-specific clinical presentation and the frequently negative radiological investigations. Displacement of the superior mesenteric artery (SMA) with crowding, stretching and engorgement of its visceral branches, mesenteric swirl and a clustered appearance of small bowel loops are CT findings with a high specificity but low sensitivity. A high degree of vigilance, communication between radiologists and surgeons and a low threshold for surgical exploration is therefore necessary.