The ureters in neonates and infants can be anatomically and functionally so severely compromised to warrant the term "dysplasia". The clinicopathologic criteria for this diagnosis are: 1) a variable degree of dilatation on the excretory urogram and/or cystogram. 2) poor or absent ureteral motility as observed on cine-fluoroscopy or at operation, 3) poor muscularization and decreased muscle/connective tissue ratio under light microscopy and 4) small and deformed muscle cells with nexuses that are markedly decreased or absent and intracellular organelles that are poorly defined under electron microscopy. Excessive collagen and ground substance are present throughout the ureter. Although these qualitative and quantitative structural abnormalities are non-specific and can occur as a result of prenatal obstruction and postnatal infection, their severity in early postnatal life and their frequent association with dysplastic kidneys suggest congenital dysplasia as their etiology.