Background/Aims: The aim of this study was to examine the relationship between the degree of urinary copper excretion and stages of diabetic nephropathy. Methods: Copper, ceruloplasmin and albumin concentrations were measured in serum and urine samples from 41 type 2 diabetic outpatients with different stages of nephropathy and from 10 healthy controls. The copper/albumin and copper/ceruloplasmin ratios in serum and urine were determined. Furthermore, we examined whether free copper ions are dissociated from ceruloplasmin under various pH conditions. Results: Urinary copper concentrations significantly increased only in macroalbuminuric patients. The copper/ceruloplasmin and copper/albumin ratios in urine were consistently greater than those in serum which were not different between patients and healthy controls except the copper/albumin ratio in macroalbuminuric patients. The ratios in urine decreased in parallel with the progression of nephropathy. Copper was found to be released from ceruloplasmin under acidic conditions. Conclusion: Urinary copper excretion in healthy controls may be the result of dissociation from the albumin-copper complex of serum during its passage through the kidney. In diabetic patients with advanced nephropathy, urinary copper excretion may be due to dissociations from both copper-albumin and ceruloplasmin-copper complexes filtered through the damaged glomerulus. Overloading of urinary copper to damaged renal tubules may play some roles in the progression of nephropathy in patients with advanced nephropathy.