Renal tubulointerstitial injury plays an important role in the development of IgA nephropathy (IgAN), the most common form of glomerulonephritis. Few currently in use biomarkers can sensitively detect the earliest signs of renal tubular injury, hindering our efforts to launch preventive and therapeutic measures for this disorder in a timely manner. Neutrophil gelatinase-associated lipocalin (NGAL) is an acute phase protein that is rapidly released from not only neutrophils but also a variety of cell types upon inflammation and tissue injury. Its small molecular size and protease resistance could render it an excellent biomarker of renal injury in IgAN. In this study, we tested this hypothesis by measuring urinary levels of NGAL, creatinine and N-acetyl-beta-D-glucosaminidase (NAG) in 40 healthy individuals and 70 IgAN patients with various disease severities. The urinary NGAL levels and NGAL/creatinine values were significantly upregulated in Lee grade III IgAN patients, in correlation with progressive glomerular mesangial proliferation and tubulointerstitial injury. Compared with urinary NAG levels, the urinary NGAL levels elevated much more drastically and can be readily detected even in Lee grade II IgAN patients when their NAG levels showed almost no change. Our findings suggest the promising use of urinary NGAL as an early biomarker for tubulointerstitial injury of IgA nephropathy and perhaps other types of renal disease in general.