Aims/hypothesis. To identify factors associated with early development of and late protection from microvascular complications in subjects with Type I (insulin-dependent) diabetes mellitus.¶Methods. The frequency of microvascular complications and their relation to risk factors were studied in 300 Type I diabetic subjects with short duration of disease ( ≤ 5 years) compared with 1062 subjects with long duration ( ≥ 14 years). Microvascular disease was defined as the presence of either retinopathy (assessed from centrally-graded retinal photographs) or urinary albumin excretion rate of more than 20 μg/min.¶Resu1ts. The prevalence of microvascular disease was 25 % in the short duration group. In the long duration group 18 % had no evidence of microvascular complications. In the short duration group factors associated with early development of complications were cigarette smoking and a family history of hypertension. Subjects free of microvascular complications in spite of long duration of diabetes had better glycaemic control, lower blood pressure, better lipid profile and lower von Willebrand factor levels.¶Conclusion/interpretation. At the early stages of Type I diabetes, cigarette smoking and genetic susceptibility to hypertension are important risk factors for microvascular complications. At a later stage, additional risk factors are poorer glycaemic control, higher blood pressure, and an unfavourable lipid profile possibly associated with endothelial dysfunction. Many of these factors are amenable to long-term intervention which should be started as soon as possible in the course of the disease. [Diabetologia (2000) 43: 348–355]