Urinary obstruction refers to partial or complete inability to pass urine. Flow of urine may be obstructed anywhere in the urinary tract, such as in the kidneys, ureters, bladder, or urethra.

There are many causes of urinary obstruction, including kidney disease, calculi (stones), casts, blood clots, tumors, injuries, scar formation from infection or surgery, enlargement of the prostate gland, and shock.

Urinary obstruction causes retention of waste products in the bloodstream, possible irreversible kidney damage, urinary tract infections, and a multitude of injurious changes in vital body chemistry that may lead to death.

Complete inability to pass urine is an emergency situation requiring immediate treatment. Reduced urine flow often goes unnoticed until the problem is advanced, and the animal begins to show signs of uremic poisoning, such as depression and vomiting. Reduced urine flow is not an emergency, but it can proceed to complete urinary obstruction; therefore, the animal should be examined by a veterinarian as soon as possible. Cats that frequently visit the litterbox and dogs that frequently wish to go outdoors to urinate should be examined by a veterinarian.