Previously we have shown that relative glomerular mesangial expansion was an important correlate of renal dysfunction in diabetes. To extend the understanding of structural functional relationships, 37 patients with type I diabetes mellitus for 5 to 33 years were studied with multiple creatinine clearance (Ccr), urinary albumin excretion, and blood pressure measurements, and percutaneous renal biopsies. Glomerular volume and percent sclerosed glomeruli were determined; quantitative stereology was performed to determine relative glomerular structural parameters. Per glomerulus we calculated mesangial volume and capillary filtration surface and per patient we estimated capillary filtration surface. Capillary filtration surface per glomerulus or per patient were highly predictive of Ccr (r = +0.78, r = +0.79, P less than 0.001). There was a significant but weak relationship between Ccr and mesangial volume. However, mesangial volume and glomerular volume together were highly predictive of both Ccr and filtration surface. Mesangial volume was increased and filtration surface decreased in the hypertensive patients and the patients with urinary albumin excretion less than 250 mg/24 hr. Thus, it appears that mesangial expansion within a relatively large glomerulus has less influence on filtration than does a similar increase in mesangial volume within a smaller glomerulus. There is a striking relationship between glomerular filtration rate and filtration surface in diabetes throughout the range from hyperfiltration to significant hypofiltration.