We aimed to determine the natural history of borderline increases in albuminuria in adolescents with insulin-dependent (Type 1) diabetes mellitus (IDDM) and factors which are associated with progression to persistent microalbuminura. Fifty-five normotensive adolescents with IDDM and intermittent microalbuminura (overnight albumin excretion ratte of 20-200 g min -1 on one of three consecutive timed collections, n = 29) or borderline albuminura (mean overnight albumin excretion rate of 7.2-20 g min -1 on one of three consecutive timed collections, n = 30) were followed prospectively at 3 monthly intervals. The endpoint was persistent microalbuminuria defined as a minimum of three of four consecutive overnight albumin excretion rates of greater than 20 g min -1 . One hundred and forty-two adolescents with IDDM and normoalbuminura were also followed prospectively. Fifteen of the 59 patients (25.4 %) with intermittent (9/29) or borderline (6/30) albuminura progressed to persistent microalbuminura (progressors) over 28 (15-50) months [median (range)] in comparison with two of the 142 patients with normoalbuminuria at entry (relative risk =12.6; p =0.001). Progressors to persistent microalbuminura were pubertal and had higher systolic (p = 0.02) and diastolic (p = 0.02) blood pressure, and HbA lc (p = 0.004) than non-progressors. All patients remained normotensive. Glomerular filtration rate, apolipoproteins, dietary phosphorus, protein and sodium intakes, and prevalence of smoking did not differ between progressors and nonprogressors. Total renin was higher in the diabetic patients without a difference between progressors and non-progressors. In conclusion there is a relatively high rate of progression to persistent microalbuminuria in pubertal adolescents with borderline increases in albuminura and duration greater than 3 years. These patients require attention to minimize associated factors of poor metabolic control and higher blood pressure in the development of incipient nephropathy.