Early detection and timely intervention are important for improving contrast-induced nephropathy (CIN) prognosis. Whether urinary N-acetyl-β-glucosaminidase (NAG) is a useful marker for early detection of CIN was investigated in 590 patients undergoing diagnostic coronary angiography (CA) and/or therapeutic percutaneous coronary intervention (PCI) for acute coronary syndromes or stable angina, and who received low-osmolality nonionic contrast agent. Urinary NAG, osmolality and serum creatinine were measured before and 1, 2 and 6 days after contrast agent exposure. CIN occurred in 33 patients; its incidence in high-risk patients (pre-existing renal dysfunction with/without diabetes mellitus) was significantly higher than in others. In patients with CIN, urinary NAG and serum creatinine levels on days 1 and 2 were significantly higher than at baseline and compared with patients without CIN; mean levels were gradually returning to baseline by day 6. Compared with serum creatinine, urinary NAG levels peaked earlier in CIN patients and increased much more. The results suggest that, following CA and/or PCI, CIN occurs to a certain degree and that NAG may be a useful early CIN marker as it is noninvasive, simple, inexpensive and sensitive.