INFO/WWW-LINKS: Urothelial tumors (more than 90% are malignant - transitional-cell carcinoma) occur commonly in the bladder and the renal pelvis. Ureteric tumous in the ureters are rare. In vesicoureteric reflux you may disinguish two kinds of obstruction: Bilateral obstruction and unilateral obstruction. The first is caused by urethral strictures, urethal valves, prostatic hypertrophy, bladder tumors, blood clot, calculi and pelvic malignancy. Unilateral obstructions may be caused by calculi, tumors, pelvi-ureteric junction obstructions, retroperitoneal fibrosis, compression by lymph nodes and pelvic malignancy.