Aims: There is increasing evidence that O-linked N-acetylglucosamine (O-GlcNAc) plays an important role in cell signaling pathways. It has also been reported that increases in O-GlcNAc contribute to the development of diabetes and diabetic complications; however, little is known about O-GlcNAc levels in diabetic nephropathy (DNP). Therefore the goal of this study was to determine whether O-GlcNAc could be detected in human kidney biopsy specimens, and if so to examine whether O-GlcNAc levels were increased in the kidneys of patients with DNP compared to the non-diabetic individuals. Main methods: Kidney biopsy specimens were obtained from type-2 diabetic patients (n = 6) and patients diagnosed with thin basement membrane nephropathy (n = 7) were used as non-diabetic controls. O-GlcNAc levels were assessed by immunohistochemistry using the anti-O-GlcNAc antibody CTD110.6. Key findings: We show that O-GlcNAc modification of proteins can be detected in the human kidney biopsy specimens. Furthermore, in diabetic patients, we found significantly increased numbers of O-GlcNAc positive cells in the glomeruli and significantly elevated staining in the tubuli (both in the nucleus and in the cytosol). In addition we also observed an intense, granular O-GlcNAc staining specifically in diabetic tubuli. Significance: In light of the increase in O-GlcNAc staining in the diabetic patients, we propose that increased O-GlcNAc levels might contribute to the development of diabetic nephropathy. © 2009 Elsevier Inc. All rights reserved.