Non-enzymatic glycosylation (glycation) involves both circulating proteins, such as albumin and structural proteins, such as the components of the glomerular basement membrane. Glycated albumin is more anionic than unmodified plasma albumin at physiologic pH. Preferential urinary excretion of glycated proteins has occasionally been reported in diabetes. We therefore investigated the selectivity index (renal clearance of non-glycated/glycated albumin) in 25 insulin-dependent diabetic patients (17 with microalbuminuria and 8 with macroalbuminuria), and 19 healthy subjects. The selectivity index was significantly higher (p less than 0.01) in the microalbuminuric patients than in the macroalbuminuric patients and the control subjects: 1.11 +/- 0.03 SEM vs 0.94 +/- 0.08 vs 0.98 +/- 0.02. These results are not consistent with preferential urinary excretion of glycated albumin in insulin-dependent diabetic patients with increased urinary albumin excretion.