In children with renal tubular disorders the existence of retarded histological features of glomerular maturation has been suggested by our group. However, no valuable information is available on the frequency of histopathologically immature glomeruli in the normal kidney. For this reason we established a simple, semiquantitative definition of postnatal glomerular development: immature glomeruli (with at least half of the circumference of capillary loops densely lined with cuboidal epithelial cells), intermediate glomeruli (circumference of capillary loops lined with at least five adjoining cuboidal cells), and mature glomeruli. This definition was applied in a set of 71 normal kidneys from ages birth to 5 years. The relative frequency of the mentioned stages of glomerular maturation was strongly age dependent. In comparing the patients data with the normal findings it was possible to separate patients with the finding of disproportionately high fractions of immature glomeruli, provided the right age at biopsy is chosen. It is therefore concluded that the previous suspicion of late glomerular maturation may be validated in at least in some bioptic specimens.