A quantitative study of dimensions of immature human glomeruli and proximal convoluted tubules has been made by means of microdissection. Normal kidneys from 23 subjects varying in age from full term to 18 years were utilized in this study. The proximal convoluted tubules in the normal kidney of a full-term infant are small in relation to their corresponding glomeruli. With the rapid growth of the proximal tubules in the first few months of life, a correspondingly rapid shift in anatomic glomerulotubular balance occurs. The final dimensional change in the maturation of a proximal convoluted tubule is an increase in its volume. The extreme heterogeneity of nephrons at term is recorded, along with their rapid attainment of relative homogeneity at an early age. The best anatomic correlates for glomerulotubular balance in immature human kidneys were found to be glomerular surface area and proximal tubular volume. The small proximal tubular mass shown to exist in early infancy may well account for the low Epah, TmPAH, and TmG values which have been demonstrated during this age period.