We studied the relationships of renal lesions with clinical features and renal prognosis in 128 Japanese patients with noninsulin-dependent diabetes mellitus (NIDDM). Diabetic glomerulosclerosis (DMGS) was found in 108 cases (84.4%), and DM-associated glomerulonephritis (GN), IgA nephropathy and membranous nephropathy, was in 20 cases (15.6%). There was no significant difference in age at renal biopsy, age at DM onset, DM duration, glomerular filtration rate (GFR), urine protein and serum IgA level between patients with DMGS and DM-associated GN. With respect to histological parameters, the increase in mesangial matrix was more closely related to DM duration, GFR and urine protein than to the degree of glomerular sclerosis. The index of glomerular lesion (IGL), which was calculated by combining these two lesions, had a more significant correlation with the clinical features than the tubulo-interstitial lesions. We suggested that the mesangial change was an initiating reaction, and the interstitial one was secondary in NIDDM patients. The prognosis of renal function after renal biopsy of the patients whose serum creatinine level was less than 1.2 mg/dl, was poorer in cases with nodular lesion than in those with diffuse lesion. The grade of tubulo-interstitial lesion was also more severe in these patients with nodular lesion. However, the controls of blood glucose, blood pressure, serum total cholesterol were not different among the two groups. On the other hand, serum creatinine level of most patients with associated GN did not change for a long period, up to 10 years.(ABSTRACT TRUNCATED AT 250 WORDS)