In this prospective study of 11.9 years duration (range 9 to 14), we examined the progression of albuminuria prior to and after the onset of microalbuminuria [albumin excretion rate (AER): 20 to 200 μg/minute]. Glycated hemoglobin (HbA1), AER and blood pressure were measured every six months. Twenty-two (13 type I, 9 type II) patients were identified in whom AER increased progressively (progressors). These patients were compared with 22 others matched for age, duration and type of diabetes in whom AER did not change significantly during the study period (non-progressors). In the progressors, the rate of increase in AER correlated with mean HbA1 for the study period in patients with type I (r = 0.68, P < 0.01) and type II diabetes (r = 0.71, P < 0.05). Furthermore, AER began increasing well before the conventional 20 μg/minute threshold of microalbuminuria had been reached and within the first five years of diagnosis of type I diabetes. We conclude that in predisposed diabetic patients, long-term glycemic control is correlated with the rate of development of early renal abnormalities. Repeated measurements of AER from the time of diagnosis may be useful in the early detection of patients who will develop microalbuminuria and ultimately overt diabetic nephropathy.