Patients with Type 1 (insulin-dependent) diabetes without proteinuria were studied to define those patients who will later develop persistent proteinuria (more than 0.5 g protein/24 h). Two investigations were performed; 71 patients were studied longitudinally for 6 years and another 227 patients were studied cross-sectionally. All were less than 50 years of age and had developed diabetes before the age of 40 years. At entry into the study they had no proteinuria (Albustix method), had normal blood pressure and urinary albumin excretion rates < 200 μg/min (normal ⩽ 20 μg/min). The best predictor of persistent proteinuria or an albumin excretion rate > 200 μg/min was the initial urinary albumin excretion rate. During the longitudinal study, seven patients with an urinary albumin excretion rate of more than 70 μg/min at the start of the study developed persistent proteinuria or an albumin excretion rate > 200 μg/min. In contrast, only three out of the remaining 64 patients with urinary albumin excretion rate ⩽ 70 μg/min developed urinary albumin excretion rate > 200 μg/min. Patients with an urinary albumin excretion rate > 70 μg/min are thus at risk of developing diabetic nephropathy. We designate this stage of renal involvement incipient nephropathy. Patients with incipient nephropathy were further characterized in the cross-sectional study. Compared with normoalbuminuric patients, patients with incipient nephropathy had increased systolic and diastolic blood pressure, but normal serum creatinine. The glomerular filtration rate was higher than normal in patients with incipient nephropathy though not different from that of normoalbuminuric patients.