A 52-year-old patient presented with a ruptured abdominal aortic aneurysm after bicycle trauma. He was treated with a vascular prosthesis. His postoperative recovery was complicated by acute renal failure with anuria for which he was commenced on dialysis. His main persistent symptoms were severe abdominal pain, nausea and vomiting as well as massive ascites. Despite several attempts of a diagnostic and therapeutic ascitic tap, we were initially unable to make a diagnosis. Following each attempted paracentesis, symptoms initially improved. Ascites did reaccumulate, however, and we had to continue with his dialysis. Measurement of creatinine in the ascitic fluid was the key to the correct diagnosis. The ascitic fluid creatinine was nearly 3 times higher than the serum creatinine. The consequent MRI scan of the abdomen with excretion urogram demonstrated a leakage of the left ureter at the junction of the proximal and the middle third of the ureter with contrast leaking into the surrounding fluid.