BACKGROUND Biloma is the collection of bile outside the biliary tree as a result of visceral perforation. The most common site of disruption is the gallbladder, whereas common bile duct lesions usually occur following medical procedures or trauma. Spontaneous perforation of the common bile duct has been previously reported in the literature. Retroperitoneal biloma secondary to spontaneous perforation of the common bile duct is an extremely rare pathological entity. The purpose of this report is to inform clinical doctors of this rare entity, which can have fatal consequences for the patient. CASE REPORT We present the case of an 89-year-old man who was hospitalized with symptoms of vomiting, nausea, fatigue, and diffuse abdominal pain. The clinical examination and blood tests revealed peritonitis, a finding which was confirmed by the computed tomography of the abdomen as a retroperitoneal fluid collection, extending from the region posterior to the duodenum and head of the pancreas to the right inguinal fossa. As the patient's clinical status deteriorated, an urgent laparotomy was performed, revealing the presence of retroperitoneal biloma secondary to spontaneous perforation of the common bile duct. The operation was never completed as the patient died during the operation. CONCLUSIONS The diagnosis of this entity is difficult and is made during surgery. A large spectrum of treatment approaches has been used, but, regardless of the method, the goal is to halt the spreading abdominal contamination with bile and to treat the associated biliary pathology.