The possibility of missing the diagnosis of focal segmental glomerulosclerosis (FSGS) has been primarily attributed to the focal distribution of the sclerotic lesions, but this assumption has not been verified by any serial morphometric analysis of renal biopsy specimens. The aim of this study is to assess the size and the distribution of sclerotic lesions in primary FSGS and to establish the minimum number of glomeruli and sections necessary for the diagnosis. Fourteen biopsies from adult nephrotic patients with primary FSGS were carefully selected from a group of 41 biopsies, to minimize the possibility of finding and misinterpreting nonspecific glomerular scars, and were serially cut to obtain 1485 consecutive 2 microns-thick sections that, after PAS staining, showed 182 glomeruli. Fifty-seven glomeruli were "complete", i.e., they emerged after the first section and disappeared before the last section. The percentage of glomeruli with sclerotic lesions was 31.5% in the starting section, 71.8% after the observation of all serial sections, and 81.7% when only the complete glomeruli were considered. The morphometric analysis on complete glomeruli revealed that the volume of the sclerotic lesions averaged just 12.5% +/- 2.2 SE of the entire glomerular volume, and the statistical analysis revealed that the minimum number of glomeruli needed in the starting section to exclude sclerotic lesions is eight (P < 0.01) or nine (P < 0.001). If fewer glomeruli are seen, it is necessary to cut 2 microns-thick serial sections, but to examine just one of every 11 (P < 0.001), the number of sections to examine being proportional to the number of glomeruli found. In conclusion, this study shows that the distribution of sclerotic lesions in primary FSGS is not focal, but diffuse; however, because of the small size of the sclerotic lesions, the probability of missing the diagnosis is statistically relevant when fewer than eight glomeruli are found in the starting section, unless a serial morphological analysis, even on a reduced number of sections, is made.