PURPOSE: Urinary N-acetyl-beta-D-glucosaminidase (NAG) has been known to reflect the damage of proximal tubular cells in the early stages of renal disease. Recent studies have demonstrated that tubular grade predicted renal outcome better than did other histological parameters in IgA nephropathy. We evaluated the meaning of urinary NAG in relation with initial histological features and renal outcomes in early subclinical IgA nephropathy. METHODS: Among the firstly diagnosed IgA nephropathy patients from Jan 2001 to Dec 2002, 43 subjects were selected with the criteria of normal renal function and 24-h urinary protein excretion <3.5 g/day. The subjects were followed for 2 years. Pathologic lesion was graded according to HASS classification and semiquantitative scorings, from 0 to 3, were carried out for glomerular (GG), interstitial (IG), tubular (TG), and vascular (VG) lesion. RESULTS: The subjects consisted of 20 male and 23 female with mean age of 30+/-13 years, baseline blood pressure 116+/-15/74+/-10 mmHg, Cr 1.03+/-0.24 mg/dL, Ccr 88+/-19 mL/min, 24-h urinary protein excretion (UPER) 1, 790+/-1, 610 mg/24-h, urinary NAG 11.8+/-11.0 U/g cr at the time of biopsy. Hass subclass was correlated significantly with glomerular, tubular, and interstitial grades (all p<0.05). In comparison with clinical parameters, glomerular grade was significantly related with 24-h UPER (p<0.05) and tubular grade was significantly related with systolic blood pressure (p<0.05). Urinary NAG level at the time of biopsy show significant correlation with tubular grade (p<0.05). Progression of renal disease occurred in nine patients (20.9%). The patients with renal disease progression showed significantly low baseline Ccr, high 24-h UPER, and high NAG (all p<0.05). In pathological findings, tubular grade was significantly related with renal prognosis (p<0.05). In regression analysis, tubular grade was a independent predictor of renal prognosis among above four parameters showing significant differences. In survival analysis, tubular grade 0, 1 and grade 2, 3 showed significant difference in renal survival as compared to each other. The patients with baseline NAG urinary NAG above 10 U/g Cr showed significantly worse renal survival as compared with those below 10 U/g Cr (p<0.05). CONCLUSION: Tubular lesion is an independent factor associated with renal progression in these patients. Urinary NAG reflects well the degree of tubular lesion at the time of biopsy. We carefully suggest, therefore, that the measurement of urinary NAG level is helpful to estimate tubular lesion and predict renal prognosis in subclinical asymptomatic IgA nephropathy patients before they undergo renal biopsy.