Summary1In a baby, P.D., aged one month who died in renal failure, microdissection of the nephrons in the single large abnormal hydronephrotic kidney, associated with definite organic ureteral stricture in the upper ureter, revealed an extraordinary variation in structure from hypoplastic nephrons or parts thereof to the immensely hyperplastic units. These structures and their measurements are compared with those of a normal kidney and a kidney showing moderate hydronephrosis of unknown origin.2Greatest enlargement was in length and diameter of the convoluted tubules and in the diameter of the largest collecting tubules.3A gradation between almost normal elements to the grossly hypertropic nephrons, which made up the majority of the nephron population, was seen.4Localized abnormalities, scattered sporadically in a bizarre fashion along the length of a tubule were frequently seen.5Not only were there nephrons of grossly abnormal size, but also nephrons with gross abnormalities of structure. Gradations between these abnormal nephrons and isolated cystic bodies of glomerular, but also of possible tubular origin, were common.6Extensive infection in the peritubular connective tissues, without necessarily involving the continuity of tubules passing through such granulation tissues, was the usual finding.7Gast material was frequently seen within tubular lumens, more clearly in collecting tubules. These casts did not appear in any instance to block tubules completely. No collapse below nor dilatation above the site of the material was seen.8Quantitative nephron dimensions from three babes aged one month illustrate the enormous variation that can occur even at this young age.9The existence of extraglomerular blood vessels to tubules in this abnormal kidney was demonstrated.