N-Acetyl-beta-D-glucosaminidase (NAG) levels in serum and urine from diabetics have been measured over a three-year period to assess their potential as indicators of the onset of retinopathy and nephropathy. The presence of retinopathy and nephropathy was confirmed by fluorescein angiography performed at the end of the study and by proteinuria, respectively. Three groups of diabetics were investigated, those on insulin, on oral hypoglycaemics, or on diet only. There was no apparent correlation between total NAG activity in serum with the development of retinopathy, nor were serum isoenzyme variations useful in this context. However, urine total NAG activity demonstrated a striking difference between diabetics of all groups and normals. In particular the B isoenzyme doubled in diabetics. The potential use of this finding in relation to prediction of the onset of microangiopathy is discussed.