A clinico-pathological study was performed on 154 patients with diabetes mellitus to clarify the significance of glomerular hematuria. Glomerular hematuria was observed in 26 patients (16.9%), of whom 10 had complications of IgA glomerulonephritis and one had membranous nephropathy. The remaining patients (143 cases) were divided into two groups; a hematuria group (15 cases) and a non-hematuria group (128 cases). Patients in the hematuria group showed diabetic retinopathy, hypertension, massive proteinuria and the requirement for insulin therapy more often than those in the non-hematuria group (p < 0.01, p < 0.001, p < 0.001 and p < 0.01, respectively). In addition, the serum creatinine level in the hematuria group was significantly elevated compared to that in the non-hematuria group (p < 0.01). Histologically, patients in the hematuria group exhibited advanced diffuse lesions, nodular lesions, exudative lesions, microaneurysms, crescent formation, capsular adhesion and interstitial lesions more often than those in the non-hematuria group (all, p < 0.001). Furthermore, the vascular index in the hematuria group was significantly higher than that in the non-hematuria group (p < 0.001). It is suggested that glomerular hematuria in diabetic patients indicates the presence of diabetic nephropathy at an advanced stage or coexistence of primary glomerulonephritis.