Ureteropelvic junction (UPJ) obstruction is a congenital or acquired blockage of urine where the kidney connects with the ureter. It is typically caused by intrinsic stenosis of the upper ureter, abnormalities in surrounding blood vessels, infections, scar tissue, or kidney stones.

UPJ obstruction is the most common cause of prenatally diagnosed hydronephrosis. It can occur bilaterally but is more frequently seen unilaterally with a left-sided preference. Common findings include abdominal mass, flank or abdominal pain, nausea, vomiting, urinary tract infections, hematuria, and failure to thrive. Some patients may be asymptomatic. Rarely, and if untreated, kidney failure may occur. Surgery may be required.