In 68 type I diabetics without permanent proteinuria, mean age 28 +/- 9 years, where diabetes was detected at the age of 14 +/- 7 years and persists for 14 +/- 8 years the urinary excretion of albumin and beta-2-microglobulin was assessed. The results were evaluated in relation to the persistence of diabetes, the blood pressure reading, family-history of diabetes and type of insulin therapy. In addition to microalbuminuria in 29% of the subjects which is a manifestation of glomerular damage, the authors detected in 58% elevated beta-2-microglobulin excretion indicating early changes of the proximal tubule. There was a relationship between microalbuminuria and "relative hypertension" which enhances albumin excretion and increases the risk of diabetic nephropathy. The relationship between microalbuminuria and a positive family-history of diabetes supports the hypothesis of a genetic background for the possible development of nephropathy. There was also a relationship with the duration of diabetes and the favourable effect of prolonged intensive insulin treatment. The clinical impact of beta-2-microglobulinuria in the diagnosis of the incipient stage of diabetic nephropathy must be tested in future investigations.