OBJECTIVE: It was suggested that the cause of microalbuminuria is heterogeneous in NIDDM. However, only a few studies are available that investigated the renal pathology in NIDDM patients with microalbuminuria. This study was undertaken to evaluate renal pathology in Korean NIDDM patients with microalbuminuria. METHODS: Fifty NIDDM patients with microalbuminuria and without retinopathy were undertaken renal biopsy. Renal pathologic findings were classified as follows: group A, near-normal finding; group B, typical diabetic nephropathology; group C, atypical patterns of renal injury (mild glomerular change with disproportionally severe tubulointerstitial lesion, arteriolar hyalinosis or global glomerular sclerosis); group D, non-diabetic renal lesion. RESULTS: Seventeen patients were classified into group A, 19 into group B and 8 into group C. Six patients had non-diabetic renal lesions and they were all confirmed to be IgA nephropathy. Fasting blood sugar and GFR were significantly higher in group B than in group A and group C respectively, and systolic blood pressure was higher in group C than in group A. CONCLUSION: Renal pathology in microalbuminuric NIDDM patients without retinopathy was heterogeneous. This may explain heterogeneous clinical meaning of microalbuminuria in NIDDM.