The structure and ultrastructure were studied of 48 specimens from cases of congenital ureteroplevic junction obstructions and primary obstructive megaureters. Under light microscopy a spectrum of findings occurs, extending from the nearly normal to the clearly abnormal obstructive segments. However, under electron microscopy there were consistent abnormalities: 1) excessive collagen fibers between and around the muscle cells and 2) a group of compromised muscle cells proximal to the collagenous segment. These findings are responsible for functional discontinuity via the nexus and indistensibility of the pathologic areas. The high resolution of the electron microscope allows clearer definition of these obstructions and the impact of these findings on surgical remodeling is discussed.