A 51 years old male was admitted to our Emergency Department because a severe pain in the right lumbar area irradiating to homolateral thigh, with fever. Abdomen and pelvis TC scan revealed a big retroperitoneal abscess. An urgent laparotomy was performed. It showed a huge retroperitoneal purulent collection extending caudally to right thigh and all around urinary bladder. The fluid collections were drained and a sample was sent for biological examination. There was the evidence of enterococcus faecalis and specific antibiotic treatment was done. The patient was discharged after 33 days in good general conditions. The aim of this study is to emphasize the difficulty in clinical diagnosis of this pathology due to the lack specific symptoms and signs, and the occasional difficulty to find the original cause of the condition. Moreover we remark the utility of CT scan and of selective collection drainage.