Three recent cases of retroperitoneal abscess provide an opportunity to review this complaint, which tends to be forgotten in this age of antibiotics. The most significant clinical sign is the discovery of an abdominal mass, and ultrasound is, today, a decisive diagnostic aid. Whatever the etiology - which is often difficult to establish - the essential part of the treatment consists in early and complete surgical drainage. This drainage may be associated with surgical treatment of the kidney, either simultaneously or subsequently, if this is required.