Backflow, in the usual context, is a general term referring to the passage of contrast medium from the pelvicalyceal system into the kidney. In a more restricted sense, it refers to such passage without actual rupture of the pelvicalyceal system itself. Peripelvic extravasation refers to the presence of contrast medium outside of the pelvicalyceal system, in the renal sinus adjacent to the pelvis. Occasionally, the extravasation may extend downward in the retroperitoneal space along the ureter or psoas muscle. There are two main possibilities to account for this condition: backflow (via a fornix) and rupture of a fornix. In either case the underlying cause is ureteral obstruction, usually secondary to a calculus, with resultant increased pressure in the renal pelvis. Clinically, this is manifest by renal colic. The most important condition to be differentiated is spontaneous rupture of the renal pelvis itself, as this calls for operation, whereas the condition reported here almost always responds to conservative therapy.