Morphometric data on glomerular size are presented on three patients with solitary functioning kidneys and one with bilateral oligomeganephronic hypoplasia. Renal biopsy of each of two patients with a congenitally absent kidney (unilateral renal agenesis) and a patient with oligomeganephronie, all with proteinuria and renal insufficiency, reveal increases of mean glomerular diameters of at least 1.75 × and mean glomerular volumes greater than 5 ×. These dimensions, which are in the range of maximal increases recorded for man, are associated in all three biopsies with focal sclerosis of the hypertrophied glomeruli. By contrast, the functionally fully-compensated solitary kidney of a patient who lost function of the contralateral kidney from acquired disease, is characterized by the absence of focal glomerulosclerosis and by glomerular enlargement of significantly lesser degree (increased mean diameter 1.24 × and mean volume less than 2 ×). These observations correlate glomerular injury with glomerular size and suggest that in the setting of reduced nephron numbers, nephron destruction via focal glomerulosclerosis may be initiated when compensatory glomerular hypertrophy has reached its limits. © 1985, National Kidney Foundation, Inc.. All rights reserved.