Electron microscopy is routinely utilized in most centers in the evaluation of native renal biopsies. Several studies, primarily from the 1960s and early 1970s, provide justification for its use. Conducted by Siegel et al. (1), the largest study evaluated 213 consecutive renal biopsies and found that electron microscopy was needed for a correct diagnosis in 11%, as well as for confirmation or additional information in another 36%. However, nearly all of these studies were conducted before the use of immunofluorescence in renal biopsy diagnosis became widespread and before several new glomerular diseases and variants were described. In light of this situation and the expense of the procedure, the routine use of electron microscopy in native renal biopsies also examined by immunofluorescence and routine light microscopy was reevaluated. From January 1996 to June 1996, 288 native renal biopsies were received, and all were evaluated by the same pathologist. Of those, 233 met criteria for inclusion in this study, which were > or = 5 glomeruli for light microscopy, > or = 2 for immunofluorescence, and > or = 1 for electron microscopy, not including globally scarred glomeruli. Light microscopy (hematoxylin and eosin, periodic acid-Schiff stains) and immunofluorescence--for immunoglobulin (Ig) G, IgA, IgM, C3, C1q, fibrinogen; kappa/lambda when needed--were evaluated on each biopsy within 48 h of receipt, and a preliminary diagnosis was recorded if possible. Electron microscopy was then performed, and a final diagnosis was made. In 50 cases (21%), electron microscopy was needed to make the final diagnosis; in two of these cases, the preliminary diagnosis was incorrect, and in 48, a firm preliminary diagnosis could not be made. In the other cases, the preliminary diagnosis was correct, but in 48 (21%), ultrastructural study was felt to provide important confirmatory data, and in eight cases (3%), an additional, unrelated diagnosis was supported by the ultrastructural findings. Diagnoses most frequently requiring electron microscopy included minimal change nephropathy, early diabetic nephropathy, membranous lupus nephritis, membranoproliferative glomerulonephritis, postinfectious glomerulonephritis, thin basement membrane nephropathy (or exclusion of this in cases of otherwise unexplained hematuria), and human immunodeficiency virus-associated nephropathy (or exclusion of it in cases of collapsing glomerulopathy). Common diagnoses usually not requiring electron microscopy included IgA nephropathy, diffuse proliferative lupus nephritis, focal segmental glomerulosclerosis (not collapsing glomerulopathy variant), pauci-immune crescentic glomerulonephritis, acute interstitial nephritis, and amyloid nephropathy. This study confirms that, as was the case 20 to 30 yr ago, electron microscopy provides useful diagnostic information in nearly half of native renal biopsies. If electron microscopy cannot be performed routinely on all such biopsies, it is recommended that tissue for ultrastructural studies be set aside in each case.