OBJECTIVE - Cigarette smoking contributes to development of diabetic nephropathy. However, long-term studies on the effect of smoking on decline in kidney function in diabetic nephropathy are lacking. We assessed the impact of smoking on progression of diabetic nephropathy in type 1 diabetic patients enrolled in a prospective observational cohort study started in 1983. RESEARCH DESIGN AND METHODS - We identified all albuminuric type 1 diabetic patients (n = 301) followed for at least 3 years, median (range) 7 years (3-14), who underwent at least yearly measurement of glomerular filtration rate (GFR) by the 51CR-EDTA plasma clearance technique (n = 8, range 3-24). In total, 192 men and 109 women were included (age [mean ± SD] 36 ± 11 years, duration of diabetes 22 ± 8 years); 271 patients were treated with antihypertensive drugs, predominantly ACE inhibitors in 179 patients. Patients were classified as smokers if they smoked more than one cigarette per day during a portion of or the entire observation period. Blood pressure, albuminuria, HbA1c, and serum cholesterol were measured every 3-4 months during the study. RESULTS - In all 301 patients, the mean (SE) rate of decline in GFR (ΔGFR) was 4.0 (0.2) ml · min-1 · year-1 during the investigation period. No difference in ΔGFR was demonstrated between nonsmokers (n = 94), ΔGFR 4.5 (0.4), ex-smokers (n = 31), ΔGFR 3.1 (0.7), and smokers (n = 176), ΔGFR 3.9 (0.3) ml · min-1 · year-1, respectively (NS). Adjustment for other risk factors for progression of diabetic nephropathy did not alter the results: smoking was not associated with ΔGFR, whereas blood pressure, albuminuria, HbA1c, and serum cholesterol were demonstrated to be independent progression promoters. CONCLUSIONS - In our study, smoking was not associated with decline in kidney function in type 1 diabetic patients with diabetic nephropathy.