Urinary N-acetyl-beta-D-glucosaminidase (NAG) is a sensitive indicator of renal tubular injury. The aim of the study is to determine the status of urinary NAG excretion in Chinese children with insulin-dependent diabetes mellitus (IDDM) but without any clinical evidence of nephropathy, and to try to find the possible associated factors of such tubular injury if any. Thirty-one children (8 males and 23 females) with IDDM who have normal serum creatinine, 24-hour urinary creatinine clearance and urinary total protein excretion were enrolled in the study. The mean age of the patients was 11.6 +/- 3.7 years. The random urinary NAG levels of the patients and the normal controls were 10.76 +/- 6.32 and 3.65 +/- 1.84 U/g Cr, respectively (p < 0.001). The diabetic patients were divided into 4 groups according to their duration of disease ( < 3, 3-5, 5-10 and 10-16 years). The random urinary NAG level of each group was significantly higher than that of the control group, but there was no statistically significant difference among any 2 of these 4 groups. No good correlation was noted in our study between urinary NAG and patients' age (r = -0.23, p = 0.21), serum cholesterol (r = 0.04, p = 0.84), insulin dosage (r = 0.13, p = 0.49), 24 hour urinary creatinine clearance (r= -0.41, p - 0.085) or urinary total protein excretion (r = -0.28, p = 0.26). However, the plasma HbAlc level correlated significantly with urinary NAG (r = 0.50, p < 0.01). We concluded that urinary NAG is increased in Chinese IDDM children without any clinical evidence of nephropathy. Such increased excretion is correlated with the plasma HbAlc level. Our results suggests that there is tubular dysfunction in the early stage of IDDM children even before there is any clinical evidence of nephropathy, and urinary NAG may reflect glycemic control in such patients.