A urinoma is a mass formed by encapsulated extravasated urine. It may follow closed renal injury, surgical operation or arise spontaneously in the presence of obstruction. The essential factors are continued renal function, rupture of the collecting system and distal obstruction. The extravasating urine is localised within the perirenal fascia and stimulates an intense fibrous reaction which forms a thick wall. Early diagnosis is important for successful treatment. The clinical features include malaise, vague abdominal pain, weight loss and a palpable mass. Plain films usually reveal a mass with loss of the retroperitoneal landmarks. High-dose excretion urography usually shows renal displacement, hydronephrosis and extravasation of contrast medium into the urinoma. Should renal function be inadequate for diagnosis, renal puncture, preceded by ultrasound examination, is the investigation of choice.