Urinary ascites in a newborn infant is unusual and most commonly indicates a disruption to the integrity of the urinary tract. The following report describes a case of urinary ascites, probably due to bladder rupture caused by umbilical artery catheterization, associated with hyponatremia, hyperkalemia and elevated serum creatinine. This unusual biochemical profile is characteristic of urinary 'autodialysis' and was corrected by bladder drainage.