We studied the urinary elimination of two indicators of renal proximal tubular function. N-Acetyl-Gluosaminidase (NAG) and beta 2 microglobulin (beta 2m) in 23 healthy and 67 diabetic children aged 2-15 years and with disease evolution times between 1 month and 12 years. In order to obviate the variations of glomerular filtration present in the diabetes, both proteins were evaluated in the form of indices of excretion (IE). It was found that the diabetic children as a whole presented higher values of NAGIE than the controls (p less than 0.001), and that this increased as the degree of metabolic control worsened, the latter being judged by either fructosamin or HbA C (r = 0.59 for both). The increase in the urinary elimination of NAG had a directly proportional relation with the evolution time of the diabetes. With respect to the beta 2mIE, no significant differences were found on dividing the children according to the degree of metabolic control, although the values did increase with longer evolution times. Thus it may be concluded that NAGIE was shown to be a more useful parameter than beta 2m for evaluating the renal proximal tubular function in infant and juvenile diabetes.