The diagnostic significance of urinary sialic acid (SA) determinations was evaluated in relation to the histological findings of renal biopsy specimens. The subjects enrolled in this study comprised 82 diabetics. They were divided into 4 groups according to Gellman's criteria, namely D0, DI, DII and DIII approximately IV. Thirty non-diabetic healthy volunteers were used as controls. The urinary SA was measured by high performance liquid chromatography, and the urinary albumin excretion was estimated by solid phase radioimmunoassay. In addition, urine samples were assayed for N-acetyl-beta-D-glucosaminidase (NAG) and beta 2-microglobulin (beta 2MG). The urinary level of total SA (under conditions of hydrolysis) was significantly increased in the DII and DIII approximately IV groups as compared to the controls; however, a similar value was observed in the D0, DI and control groups. The urinary level of glycoprotein-bound SA was significantly increased in all diabetics as compared to the control group, and was significantly higher in the DII and DIII approximately IV groups than in the D0 and DI groups. The bound-SA/total-SA ratio (B/T ratio) showed a significant increase with respect to the progress of diffuse lesions. A weak correlation was noted between the B/T ratio and urinary protein excretion. However, there was no correlation between the B/T ratio and other indices. The urinary SA is considered to represent a useful indicator for estimating diabetic nephropathy.