The incidence of sclerotic glomeruli as a function of age in kidneys from 122 patients without clinical evidence of renal disease or hypertension was estimated by histologic quantitation. Based on statistical analysis of data from this sample, 95% of the normal population up to 40 years of age would be expected to have less than 10% sclerotic glomeruli. After the age of 40 years, the upper limit containing 95% of the normal population exceeds 10% sclerosis, and after the age of 50, there is a broad scatter of observed percentage of sclerotic glomeruli. These findings suggest that, in patients 40 years of age and younger, sclerosis of glomeruli at an incidence greater than 10% is disease-related, while in patients older than 40 years (and particularly those older than 50), there is a transition, and the distinction between abiotrophic involutional sclerosis and disease-related sclerosis becomes less clear.