We quantitated glomerular and cortical interstitial structures in nine type I mesangiocapillary glomerulonephritis (MCGN) patients aged 6 to 20 years whose creatinine clearance (CCr) was 10 to 129 ml/min/1.73 m,2 as compared to age-matched normal controls. Mean glomerular volume and mesangial volume fraction [Vv(mes/ glom)] were increased and the percentage of the capillary endothelial circumference which was defined as filtration surface was decreased in type I MCGN patients. Vv(mes/glom) was inversely related to filtration surface area per glomerulus (r = -0.73, P < 0.05) and directly to volume density of cortical interstitium [Vv(int/cortex)] (r = +0.90; P < 0.01). Vv(mes/glom) (r = -0.87; P < 0.01), filtration surface area per glomerulus (r = +0.83; P < 0.01) and Vv(int/cortex) (r = -0.86; P < 0.01) were correlated with CCr. Thus, in type I MCGN, measures both of glomerular and of cortical interstitial structure are highly correlated with glomerular function.