Ureteral obstruction with subsequent hydronephrosis is clinically common. Most cases are identified and diagnosed in the perinatal period. The diagnosis of ureteropelvic junction obstruction (UPJO) implies a functionally significant impairment of the urinary transportation from the renal pelvis to the ureter. Although most cases are probably congenital in origin, they can clinically remain hidden until much later in life. UPJO is usually considered an isolated event. Recently, we have evaluated a father and his 3 sons, all of whom had UPJO. This study reports a missense mutation of threonine 386, which was replaced with alanine in Wilms' tumor genes. We suggest that UPJO might not necessarily be sporadic and other family members might have a similar problem.