Retroperitoneal lipomas are a heterogeneous group of mesenchymal tumors. They are usually large and occur most frequently in the retroperitoneal, perineal and pelvic regions. Lipomas grow slowly surrounding the retroperitoneal and pelvic organs, with a displacement of bowel and vascular axis. A case of a 61-year-old male patient which referred urinary frequency, urgency and nocturia is presented. Urodynamics evidenced a detrusor instability in a low capacity bladder. CT scan demonstrated a bladder dome compression due to a huge retroperitoneal mass extending from the right hepatic lobe to the hypogastric region and the right thigh. Surgical complete resection was performed: histology demonstrated a lipoma with areas of well differentiated myxoid degeneration. After surgery the irritative urinary symptoms disappeared. This is the first case described in literature of detrusor instability due to bladder compression by retroperitoneal lipoma.