The minimum sample size for semiquantitative and quantitative evaluation of interstitial tubular and glomerular parameters is calculated. For the majority of parameters a renal cortex area of 2 mm² or 7 glomeruli (adults) or 13 glomeruli (children) is sufficient. Larger biopsies or more glomeruli are necessary for the evaluation of the area density of interstitium patent and obsolescent glomeruli as well as the numerical density of obsolescent glomeruli. The most sensitive parameters are those concerning crescents which require biopsies of 11–19 glomeruli in adults and 40 glomeruli in children. In summary, biopsies of usual size (10 mm²) containing about 20 glomeruli are sufficient for qualitative and quantitative evaluation.