The urinary excretion of retinol-binding protein (RBP) was studied in 101 insulin-dependent diabetic patients allocated to three groups according to 24-h urinary albumin excretion rate (UAE) (median of three urine collections): group 1 (n=45), normal UAE<30 mg/24h;="" group="" 2="">n=27), microalbuminuria (UAE 30–300 mg/24 h); and group 3 (n=29), clinical diabetic nephropathy (UAE>300 mg/24 h). We used 23 healthy subjects as controls. Fractional clearance of RBP (FC-RBP) and its 24-h urinary excretion rate (URBP) were higher in each diabetic group than in healthy subjects, the highest values being found in group 3. Groups 1 and 2 did not differ in URBP and FC-RBP. There was a correlation between FC-RBP and haemoglobin A1c in both the total diabetic cohort (P<0.001) and="" in="" diabetic="" patients="" in="" groups="" 1="" and="" 2="" with="" a="" glomerular="" filtration="" rate="" of="" more="" than="" 90="" ml/min="">P<0.05). no="" correlation="" was="" found="" between="" fc-rbp="" and="" uae="" and/or="" duration="" of="" diabetes="" in="" any="" of="" the="" diabetic="" groups.="" we="" conclude="" that="" the="" increased="" urinary="" excretion="" of="" rbp,="" indicating="" proximal="" tubular="" dysfunction,="" is="" already="" present="" in="" normoalbuminuric="" insulindependent="" diabetic="" patients="" and="" correlates="" with="" metabolic="" control.="" further="" deterioration="" in="" proximal="" tubular="" function="" was="" not="" observed="" in="" microalbuminuric="" patients,="" but="" is="" a="" late="" event="" in="" clinical="" diabetic="">