The overnight urinary albumin excretion rate (AER) of 87 patients with insulin-dependent diabetes mellitus was measured in 1966-67, 14 years later information was obtained on 63 of the original cohort; those alive were restudied, and for those who had died relevant clinical information and case of death were recorded. The development of clinical diabetic nephropathy ('Albustix'positive proteinuria) was related to the 1966-67 AER values. Clinical proteinuria developed in only 2 of 55 patients with AER below 30 microgram/min but in 7 of 8 with AER between 30 and 140 microgram/min. The risk of clinical diabetic nephropathy in the latter group was twenty-four time higher than inthe former. 9.1% of patients with AER below 30 microgram/min had died, compared with 37.5% with higher AER. The two groups did not differ significantly in age, sex composition, and initial blood pressure. Mean duration of diabetes was longer, but not significantly so, in those with AER above 30 microgram/min. Thus, elevated levels of microalbuminuria strongly predict the development of clinical diabetic nephropathy. These levels of AER are potentially reversible, and their detection and treatment may prevent diabetic renal disease.