In 18 control subjects and in 41 Type 1 (insulin-dependent) diabetic patients (13 with normal proteinuria, group A; 15 with microproteinuria, group B; and 13 with clinical proteinuria, group C), mean blood glucose, glycosylated haemoglobin and non-enzymatic glycosylated serum and urinary proteins, expressed as 5-hydroxymethylfurfural (5-HMF), were measured. In each group of diabetic patients, the levels of mean daily blood glucose, glycosylated haemoglobin and serum 5-HMF/mg protein were higher than in the control subjects. The urinary 5-HMF/mg proteinuria and the urinary/serum 5-HMF concentration ratio values were raised in group A and reduced in groups B and C. Moreover, they showed a negative correlation with 24-h urinary protein excretion in the control subjects and in each group of diabetic patients. The urinary 5-HMF/day in groups A, B and C was greater than in the control subjects. The urinary 5-HMF/day did not correlate with the mean daily blood glucose levels and, only in group A, did it correlate with serum 5-HMF and glycosylated haemoglobin. This suggests that, in this group, functional factors result in the increased renal elimination of 5-HMF and, therefore, of non-enzymatically glycosylated proteins. However, in the other groups of patients, this elimination depends on the degree of proteinuria.