OBJECTIVE: The development of persistent proteinuria is reported to be uncommon after 30 years of type I diabetes, and the significance of microalbuminuria in patients with long-duration diabetes is unclear. We therefore undertook to study, in detail, renal function in patients with long-duration type I diabetes. RESEARCH DESIGN AND METHODS: We measured albumin excretion rate (AER), glomerular filtration rate (GFR), and serum creatinine in 140 patients with ≥30 years of type I diabetes. We also assessed patients for other micro- macrovascular complications of diabetes together with factors implicated in the development of diabetic renal disease. RESULTS: Of the patients, 23% had microalbuminuria (AER 20–200 μg/min and/or urinary albumin-to-creatinine ratio [ACR] > 2.5 mg/mmol [men] or > 3.5 mg/mmol [women] and dipstick negative for proteinuria), 4% had overt nephropathy (AER > 200 μg/min or persistent dipstick positive proteinuria), and 73% were normoalbuminuric (AER < 20 μg/min or ACR < 2.5 mg/mmol [men], < 3.5 mg/mmol [women]). Patients with microalbuminuria had lower GFR (81.5 ± 30.2 vs. 96.0 ± 20.3 ml · min−1 · 1.73 m−2; P = 0.06) and higher serum creatinine (104 ± 32 vs. 92 ± 16 μmol/l; P = 0.06) than those patients with normoalbuminuria. Both serum creatinine (P < 0.05) and tobacco consumption (P = 0.01) were significantly and independently related to elevated albumin excretion. CONCLUSIONS: A substantial proportion of these patients have microalbuminuria and evidence of impaired renal function, suggesting that progression to end-stage renal failure may still be inevitable. Clinicians should continue to monitor renal function in these patients. The relationship between tobacco consumption and nephropathy also makes clear the need to reduce cigarette smoking in the diabetic population.