Hip replacement surgery remains one of the most successful and common operations in modern orthopaedics. Many surgical approaches to the hip have been described. A potential anatomical weakness exists between the hip joint and the retroperitoneal space. We describe this potential space, which lies superficial to iliopsoas and its importance in hip replacement surgery. The clinical relevance of this space is illustrated by 2 cases of retro-peritoneal migration of prosthetic femoral heads and the consequences of these.