The renal biopsy specimens obtained from 15 adults with focal and segmental glomerulosclerosis (FSGS) and 15 adults with minimal change nephrotic syndrome (MCNS) were morphometrically analyzed using light and electron microscopy. Moreover, initial biopsy specimens obtained from 10 adults who were diagnosed as MCNS and developed FSGS, based on repeat biopsy findings (‘MCNS’-FSGS), were also analyzed using electron microscopy. After comparing the actual values between the groups of FSGS and MCNS, the mean glomerular volume, the capillary volume per a glomerulus, the capillary filtration surface per a glomerulus, and the capillary diameter (Cap-D) were all larger in the FSGS group than in the MCNS group. In regard to the morphological values in the ‘MCNS’-FSGS group, both values of the surface density of the capillary filtering surface and the capillary diameter at the first biopsy specimens as well as those in the FSGS group were higher than those in the MCNS group. When analyzing the structural parameters, in the FSGS series, we found a high association of the percentage of obsolescent glomeruli (%SG) with the mean glomerular volume, the capillary volume per a glomerulus, the capillary filtering surface per a glomerulus and the capillary length per a glomerulus, however we failed to demonstrate the correlation between the %SG and the Cap-D. Thus, the glomerular structure in the ‘MCNS’-FSGS patients, even at the first renal biopsy, resembled that in FSGS, suggesting FSGS to be a distinct entity from MCNS. These data indicate that the enlargement of the capillary volume, resulting from the widening of the capillaries, was the initial structural event for adults with FSGS, while the elongation of the capillaries appeared to reflect some compensatory process for the decrease in the functioning nephron.