Pyelosinus backflow of urine usually occurs through infractions of the calyceal fornices from obstruction or retrograde study. The primary cause appears to be a rapid rise in intrapelvic pressure with the backflow representing a physiologic release mechanism which returns the intrapelvic pressure to normal. Pyelosinus backflow is a benign process. However, complications can occur. In some persons whose urine transport cannot accommodate periodic increased urine flow or pressure, repeated pyelosinus backflow may lead to pseudocyst (urinomas) or retroperitoneal fibrosis. Several mechanisms produce these complications in children and adults. The authors present cases of urinomas related to upper tract obstruction from posterior urethral valves as well as ureteral obstruction by tumor, post-treatment scarring, retroperitoneal fibrosis, or calculus. Intrarenal urinomas are illustrated presumably in kidneys with firm capsular attachments to the renal pelvis which do not allow retroperitoneal extravasation.