A cohort of 63 Type 1 insulin-dependent diabetic patients were first characterized for overnight urinary albumin excretion rate (AER) in 1967. In 1981, seven out of eight (87%) patients with initial AER greater than or equal to 30 less than or equal to 140 micrograms/min (microalbuminuria) developed clinical proteinuria compared to only 2 out of 55 (4%) patients with initial AER less than 30 micrograms/min. The same cohort of patients was reassessed in 1990 after a total follow-up period of 23 years. The aim was to investigate the role of microalbuminuria in the prediction of total/cardiovascular mortality and the development of renal failure, in addition to clinical proteinuria. The initially microalbuminuric patients had a significantly higher risk of developing not only clinical proteinuria (relative risk 9.3, 95% C.I. 1.36 to 3.10, P less than 0.05), but also of dying from a cardiovascular cause (relative risk 2.94, 95% C.I. 1.18 to 7.34, P less than 0.05). The rate of progression to renal failure was higher but not significantly so in the microalbuminuric (2 of 8) compared to the normoalbuminuric (4 of 53) group (relative risk 3.31, 95% C.I. 0.72 to 15.24, NS). In insulin-dependent diabetic patients microalbuminuria is a powerful predictor of clinically overt diabetic renal disease as well as cardiovascular mortality.