Aim: We tried to establish the significance of quantifying urinary type IV collagen (W-C) excretion for the evaluation of renal involvement of type 2-diabetic patients. Methods: Twenty patients (13 males and 7 females, age range 31 to 69 years) with type 2 diabetes mellitus had undergone renal biopsy and relationship between the severity of morphological alteration, IV-C expression and urinary IV-C excretion were examined. Results. Urinary IV-C excretion significantly correlated with mesangial expansion score (rho = 0.49, p < 0.05) and tubulointerstitial injury score (rho = 0.56, p < 0.05). Furthermore, urinary IV-C excretion significantly correlated with both glomerular (r = 0.56, p < 0.01) and tubulointerstitial IV-C expression areas. Urinary protein excretion also correlated with mesangial expansion score and tubulointerstitial injury score. However, it did not correlate with the expression of IV-C in the kidney. Conclusions: These results suggest that urinary IV-C excretion reflects the pathogenetic process of diabetic nephropathy, which urinary protein excretion alone cannot do sufficiently. It can be concluded that urinary IV-C excretion could be a more useful marker for the evaluation of renal involvement of type 2-diabetic patients.