Objective: To estimate the prevalence of microalbuminuria, overnight urinary albumin excretion rate (AER) ≥30 and ≤250 μg/min, in a large sequential sample of nonhypertensive insulin-dependent (type I) diabetic patients attending hospital diabetic clinics, to identify micro- and normoalbuminuric patients in this sample for subsequent intervention and natural history follow-up studies, and to compare the clinical characteristics of the micro- and normoalbuminuric patients identified. Research Design and methods: Screening was conducted in two phases. In phase 1, all eligible patients were asked to provide an early morning urine specimen for measurement of albumin concentration and albumin-creatinine ratio. In phase 2, all patients with an albumin concn ≥15 mg/L and/or an albumin-creatinine ratio ≥3.5 mg/mmol and a random sample of those with an albumin concn <15 mg/L and albumin-creatinine ratio <3.5 mg/mmol were asked to collect a timed overnight urine specimen for determination of AER. Results: Among 1888 patients (16–60 yr old, diabetes onset <40 yr, and duration of diabetes <35 yr) who were screened, the prevalence of microalbuminuria was ∼3.7% (95% confidence interval (CI) 2.7–7.6%). Duration of diabetes was significantly longer in micro- than normoalbuminuric patients (20 vs. 15 yr, respectively; P <0.001), and in no patient with microalbuminuria was the duration of diabetes <5 yr. Systolic and diastolic blood pressures, higher in micro- than normoalbuminuric patients (132 vs. 122 mmHg, P <0.01; 77 vs. 72 mmHg, P <0.01), were strongly associated with AER. Conclusions: Microalbuminuria in type I diabetes, which appears to represent an earlier phase in the development of clinical nephropathy, is associated with elevated blood pressure and a longer duration of diabetes.