Abstract. Prospective studies have established smoking as an independent risk factor for diabetic nephropathy, suggesting an adverse effect of smoking on glomerular structure and function. To test this hypothesis, this study evaluated GFR, metabolic profile, and smoking habits in 96 patients with type 2 diabetes and abnormal albumin excretion rate (AER). All patients underwent percutaneous kidney biopsy: mesangial fractional volume [Vv (mes/glom)] and glomerular basement membrane (GBM) width were estimated by electron microscopic morphometric analysis; interstitial fibrosis was estimated semiquantitatively by light microscopy. Forty-eight patients were smokers. Compared with nonsmokers, smokers had higher values of HbA 1c (P ϭ 0.002), AER (P ϭ 0.026), GFR (P ϭ 0.004), and GBM width (P ϭ 0.002); moreover, GFR was higher in current smokers than in former smokers (P ϭ 0.001), and GBM width was related to heavy smoking (F ϭ 5.4; P ϭ 0.006). Multiple linear regression analyses revealed that HbA 1c was associated with fasting blood glucose ( coef ϭ 0.52; P Ͻ 0.001), smoking habit ( coef ϭ 0.31; P Ͻ 0.001), insulin therapy ( coef ϭ 0.22; P ϭ 0.012), and male gender ( coef ϭ Ϫ0.20; P ϭ 0.020); AER was related to Vv (mes/glom) ( coef ϭ 0.32; P ϭ 0.003), GBM width ( coef ϭ 0.28; P ϭ 0.016), and interaction between smoking habit and HbA 1c ( coef ϭ 0.24; P ϭ 0.040). GFR was negatively correlated with Vv (mes/glom) ( coef ϭ Ϫ0.57; P Ͻ 0.001) and age ( coef ϭ Ϫ0.29; P ϭ 0.001) and positively correlated with GBM width ( coef ϭ 0.27; P ϭ 0.012), heavy current smoking ( coef ϭ 0.24; P ϭ 0.028), and HbA 1c ( coef ϭ 0.28; P ϭ 0.040); GBM width was explained by Vv (mes/ glom) ( coef ϭ 0.53; P Ͻ 0.001), interaction between heavy smoking and HbA 1c levels ( coef ϭ 0.25; P ϭ 0.003), and diabetes duration ( coef ϭ 0.23; P ϭ 0.010). Smoking habit did not affect the index of interstitial fibrosis. In conclusion, cigarette smoking affects glomerular structure and function in type 2 diabetes and may be an important factor for the onset and progression of diabetic nephropathy.Several observations suggest that the kidney, in addition to other peripheral vascular beds, is another target organ of smoking. Acute and chronic smoking has been documented to be associated with renal functional impairment, probably mediated by smoking-induced changes in vasoactive hormones (1-3). Cross-sectional and longitudinal studies have shown that smoking has an adverse effect on renal outcome in essential hypertension, primary and secondary nephropathies, and on graft and patient survival in renal transplant recipients (4 -9). In diabetic renal disease, smoking emerged as an independent risk factor for the onset of microalbuminuria, the acceleration in the rate of progression from microalbuminuria to proteinuria, and subsequent renal failure both in type 1 and type 2 diabetes (10,11).The presence of microalbuminuria is the earliest clinical marker of renal injury in diabetic nephropathy; it is therefore conceivable that the documented associati...