Renal biopsy specimens of 29 Japanese non-insulin dependent diabetes mellitus (NIDDM) patients were examined by quantitative electron microscopic morphometry. In NIDDM the relative increase of percent total mesangium and mesangial capillary surface density (S/Vb) and the relative decrease of peripheral capillary surface density (S/Va) were compared with disease controls. However, mesangial-GBM-epithelial surface density (S/Vc) was not different between both groups. These results suggest that the increased mesangial matrix expands directly towards the capillary lumen as well as along the inner surface of GBM, and narrows the capillary lumen and filtration surface. The duration of diabetes mellitus (DM) did not correlate with all morphological parameters. The mesangial expansion correlated with urinary protein excretion and decreased creatinine clearance (CCr). GBM thickening correlated with proteinuria, but not with CCr. The degree of these morphological changes could be the indicators of hypertension of NIDDM patients. Areas of thin GBM were occasionally noticed in glomeruli which revealed thick GBM extensively, although the mechanism of GBM thinning is not known at the present time.