Small amounts of urinary proteins observed at early stages of diabetic nephropathy may result from both glomerular and proximal tubular dysfunction. However, the relative contribution of the two mechanisms remains controversial. We compared the urinary excretion of three low-molecular-weight proteins (albumin, alpha 1-microglobulin and retinal-binding protein) in 104 diabetic patients and 104 control subjects who had a plasma creatinine concentration of less than 130 mumol/l. Excretion of low-molecular-weight proteins was expressed as the ratio of protein to creatinine concentration. There were significant correlation (p < 0.01) between excretion of the three low-molecular-weight proteins measured in urine sample and 24-h urine collections in diabetic and control subjects. The concentrations of these proteins in single voided urinary samples and 24-h urine collections in diabetic patients were highly correlated. Excretion of retinal-binding protein, alpha 1-microglobulin and glucose in 24-h urine collections and of RBP and glucose in urinary samples was higher in diabetic than non-diabetic patients regardless of the type of diabetes and insulin-dependence. alpha 1-microglobulin and albumin excretions in urinary samples were significantly influenced by blood glucose control, as attested by the glycosylated haemoglobin level. Increased urinary excretion of alpha 1-microglobulin and retinol-binding protein reflected proximal tubular dysfunction in diabetic patients. However, only alpha 1-microglobulin correlated with the glycaemic control. It remains to be determined whether this protein could serve as an additional early marker of diabetic nephropathy.