Urinary excretion of N-acetyl-beta-D-glucosaminidase (NAG) and retinol-binding protein (RBP), sensitive markers of renal tubular damage and dysfunction respectively, were evaluated in paired remission and relapse urine samples from 16 patients (median age 12 years), with minimal change nephrotic syndrome (MCNS), in single samples from 5 nephrotic patients (median age 12 years) with focal segmental glomerulosclerosis (FSGS) and in 183 normal controls aged 2-16 years. The NAG and RBP data were expressed as a ratio over urinary creatinine (Cr). The NAG/Cr and RBP/Cr geometric means (ranges) for normal subjects were 11.1 (3.4-35.5) micromol 2-methoxy-4-(2'-nitrovinyl)-phenol (MNP)/h per mmol and 3.1 (0.3-38.8) microg/mmol, respectively. The NAG/Cr data revealed a weak negative correlation with age in normal children, whereas RBP/Cr was independent of age. RBP/Cr and NAG/Cr in MCNS in remission were the same as in controls. In MCNS in relapse, NAG/Cr was significantly elevated (P=<0.001), while in FSGS both RBP/Cr and NAG/Cr were significantly raised (P=<0.001 and P<0.008, respectively). These findings suggest that elevated NAG/Cr may be an indicator of relapse in both MCNS and FSGS and elevated RBP/Cr may allow differentiation between the two.