Diabetes mellitus can lead, along the years of its course, to chronic renal failure in a high proportion of cases. An early risk-indicator of later diabetic nephropathy is the presence of microalbuminuria, but it usually takes about fifteen to twenty years to appear. Before that, no clinical signs can disclose the underlying alterations of glomerular basement membrane that will eventually bring forth overt nephropathy. The usefulness of the altered excretion of isoenzymes of amylase as an early marker of the glomerular charge selectivity was tested in 202 juvenile onset insulin-dependent diabetics, compared with 51 normal subjects matched for age and sex. The diabetic patients studied showed increased excretion of salivary amylase into urine. The salivary to pancreatic amylase ratio of concentrations in urine was always below 1 in normal subjects, and was increased over 1 in 33.2% of diabetics, although microalbuminuria was present only in 26.2% of patients. The excretion of other proteins was within reference values in the majority of cases, indicating that the kidney was not seriously affected in those patients. Moreover, the altered salivary to pancreatic amylase ratio in urine was more prevalent than microalbuminuria (36.6% vs 18%) in the first decade of the evolution of the diabetes. These results indicate that the ratio of excretions of both isoamylases into urine is a more sensible and earlier marker of altered glomerular charge barrier for anionic proteins.