It is important to emphasize that there is a wide range in nephron number in human subjects with apparently normal kidney morphology and renal function. This is contrary to what has commonly been reported in many textbooks, which state that the human kidney contains one million nephrons. Several studies in the past 20 years have estimated total nephron number in normal human kidneys using unbiased stereological methods (Nyengaard and Bendtsen 1992; Keller et al. 2003; Hoy et al. 2003; Hughson et al. 2003). It is reassuring that the mean values reported by these studies are relatively similar. Moreover, all studies have reported large ranges in nephron number, with the studies with the largest sample sizes reporting the largest ranges. Nyengaard and Bendtsen (1992) used the physical disector/fractionator method to estimate (count) the number of glomeruli in an autopsy study of 37 adult Danish human subjects (16-87 years of age) with no evidence of renal disease. Total nephron number ranged fourfold, from 331,000 to 1,424,000, with a mean value of 617,000. In these apparently normal kidneys, Nyengaard and Bendtsen (1992) demonstrated an age-related decline in nephron number and a positive correlation of kidney weight with total glomerular volume but not number of glomeruli.