['Interstitial expansion is important in the progression of a variety of kidney diseases, including diabetic nephropathy (DN). However, the interstitial elements that constitute interstitial expansion in DN are unknown and are the subject of this report.', 'Interstitial composition was analyzed in 15 long-standing type 1 diabetic patients, 8 with mild ( congruent with 1.5 x normal) and 7 with moderate ( congruent with 2 x normal) increases in cortical interstitial fractional volume [Vv(Int/cortex]. The mild group was 29 +/- 5 (mean +/- SD) years old with diabetes duration of 17 +/- 5 years. The moderate group was older (41 +/- 7 years; P < 0.03), had longer diabetes duration (28 +/- 7 years; P = 0.002), lower creatinine clearance (90 +/- 14 mL/min/1.73 m2 vs. 109 +/- 18 mL/min/1.73 m2; P = 0.05) and used antihypertensive medications more frequently (0/8 vs. 4/7; P < 0.03) compared to the mild group. Age- and gender-matched normal controls (N = 9) also were studied. Interstitial composition was evaluated by morphometric analysis of electron microscopic (EM) micrographs systematically obtained without bias at high (x 7500) and low (x 1500) magnification.', 'Mild interstitial expansion was associated with an congruent with 50% increase in fractional volume of interstitial cells (P < 0.001) and congruent with 70% increase in fractional volume of interstitial nuclei (P < 0.01). Numerical density of interstitial nuclei was normal in these patients, suggesting that the interstitial cells might be larger rather than simply more numerous. An increase over normal in the interstitial fractional volume of fibrillary collagen of congruent with 50% was seen only with moderate expansion (P < 0.001), when creatinine clearance was already decreased. Interstitial expansion was associated with a decrease in volume and surface of peritubular capillaries as well as with a reduction in surface ratio of capillaries to tubules.'] Does an increase in the cell component of the cortical interstitium antedate interstitial fibrosis in type 1 diabetic patients?